Logically, I think the best place to start is with defensive caps. That is, limits on active defense bonuses such as DB and Enhanced Defenses. Once you know what a good defense is, you can figure out what a good offense needs to be. But that's trickier to figure out - and both the Basic Set and my own approach has existing work to draw on for offensive limitations. So let's start there, and see where it goes. In Part II, I'll look at Defenses. In Part III, I'll pull it together and I may end up choosing different numbers than I work out today and tomorrow. We'll see. Part IV will be the DF-specific version of this all, where my current plan is to make some ties to templates for some of the rules.
I've previously done some work on offensive limitations in GURPS. What if you combine them? What kind of skill is necessary if you do so?
This post assumes using two suggested rules:
Two Tiered Fixed Deceptive Attack
Fixed Effect Feints
Short version? Deceptive Attack is either -2 (-4 to hit) (straight out of Basic Set, p. 370) or -4 (-8 to hit.) Successful feints inflict a -4 to defend, or -8 in a cinematic game (not discussed below, however.)
How High of a Skill?
What skill depends on what you can do with it.
What should someone be able to absorb in terms of penalties?
First, we need to know what we think a good skill maximum is. If we define "maximum" as "16 or less" to maximize critical hits (3-6), minimize misses (17) and critical misses (18), how much can you really need to have that skill while taking your best shot?
Since you can at most take -8 to yourself to inflict defensive penalties, we're looking at skill 24. Skill 20 if you use the Basic Set version of fixed Deceptive Attack.
If you assume Rapid Strike, that's another -6 or -3, depending on if you assume Weapon Master or Trained By A Master. I would, personally, because if you're expecting to pull off a maximally deceptive attack multiple times due to high skill, you're already effectively playing a game that assumes cinematic levels of ability. So call this -3, which means 23 or 27, depending on the fixed levels of Deceptive Attack.
How about bonuses?
There are some, but I don't think it's worth basing anying around them.
+4 Telegraphic Attack
+4 All-Out Attack (Determined)
+3 Accuracy enchantment
+1 Balanced weapon prefix
+3 Higher Purpose
+1 or +2 (DFRPG simplified relative height) . . . and not a lot of others.
Most of those are very situational, equipment based, or come with costs. It's worth putting them aside and making them truly bonus - not something that's factored in to "need."
What about other penalties?
If you want a true skill maximum, you'll need to go fairly nuts.
-10 for chinks in armor (eyes)
-2 for bad footing
-3 on fire
-3 lying down
-2 large shield
-2 striking into close combat
-6 rapid strike (without TBAM/WM)
-9 partial darkness
So, -41. You'd need Skill 57 to have a 16 or less to stab someone through the eyeslits while prone, grappled, on fire, in nearly total darkness, into close combat, while holding a large shield, and having bad footing despite not being on your feet. It's only skill 51 if you skip Rapid Strike. It's skill 61 if you're off-handed.
And that assumes you'd not using Size Modifier as a negative to melee attacks.
That's why I say it's ridiculous to set a maxim off of the worst case. It's possible to make the case above even worse if you're using Technical Grappling (like I do) or Martial Arts and fight in even less favorable circumstances.
Who wants to play in a game where penalties are basically a non-starter? That's a fun way to take dumb combats out of the equation in video games. It's less fun in a game where combat can and should be a part of the fun, and the fun is predicated on uncertainty and risk.
Lower skill maximum?
All of this assumes mastery - a 16 or less when inflicting maximum penalties on your foe, with maximum attacks, is "standard."
What if it's not? What if you standard is lower? Let's go the lowest possible - net 10 skill. Still high enough for Rapid Strike or Deceptive Attack, and you hit half the time. It's easy enough to bracket 12-15 in here.
For a 10, we're looking at skill 17 to 21. Too low, obviously - and people won't generally go for broke and shoot for a 10 skill, and it doesn't feel "highly skilled" if that's the case even without any other penalties.
Pulling it Together
The maximum penalty you can inflict on an opponent is -8, which takes two actions - a feint, and an attack. This may take two full turns, or less, depending on the Maneuver you choose, use of the rules on Martial Arts, p. 127, and any Extra Attack. Doing so also reduces your skill by -8.
A "good" skill maximum is probably around 23-27 skill, possibly more like 25-29 (call it 30 for ease of use.) That's not far off from maximum human DX of 20 and a 3e-like skill cap of DX+10. Or from a Man-to-Man era skill 25 being "Master" level skill (per MTM, p. 10)
That skill 25 nicely works with the discretionary and inflicted penalties above is nice - a skill 25 guy can hit the eyeslits on a 15 or less, or hit the skull twice on a 15 or less (12 w/o WM/TBAM), or launch a pair of very deceptive attacks on a 14. A skill 30 guy can do all of that, plus absorb lots of conditional penalties at the same time.
Nicely, though, with the limits on Deceptive Attack and Feint, that skill can't be changed into a truly massive defense-smashing attack. You can't turn that 30 into a Rapid Strike at 27/27 as a Feint-27 and 17 or less Deceptive Attack -5 that couples with the Feint. That might be a negative - I don't think so, but we'll see tomorrow with Defense. You're maxed out at a -6 or -8, depending on the Deceptive Attack level you use. This also means very skill opponents aren't beating each other with dice rolls - you don't get two skill 24 guys fighting it out and have one roll an 8 on his Feint and the other an 18 and end up with a -10 to defend, but then a turn later closer die rolls mean a -0 to -2. It also nicely limits how dangerous a foe needs to be to be dangerous to you - they don't need to be almost your skill or basically doomed from stacked penalties.
So maybe we're looking at skill 25-30 as a maximum of basically useful skill, or a stat-based DX+10. Skill caps (Part III) may be useful here, or it may just be clear that you don't need to keep skills going higher and higher and higher as the benefits start to drop off quickly. In an advantage-based game like DF, Power-Ups can eliminate penalties instead of using skill to do so; in a Martial Arts game, techniques do the job nicely.
Closing thoughts: So far, it seems like my educated guesses on levels of skill are right in the ballpark. Defense will really tell me if the limitations on inflictable penalties are "correct." If not, we'll adjust in Part III.