I'm not sure if I ever put this kind of thing up - this is how I note rooms in my dungeon files. While I like the idea of direct notation on the map, I find it's too hard to fit everything I need on the map in actual practice. Some notes go on it (and boy does that make for quick reference) but since multiple trips to a megadungeon accrue a lot of history, and since my maps are so small (8 squares to the inch), it's hard to really for me to put down sufficient useful detail.
So how to make the level key work better?
Make Your Rooms Easy to Find and Read. The goal in a megadungeon is to make it easy to find a specific room on your level key,
Use Unique Room Numbers - The map only says 1, 2, or 3, but coupled with the map label I create a unique identifier in my room key. Something off of, say, the second sub-level 5 layers down might be 5B-16, or room 8D-22 is room 22 on sublevel 8D, which is above level 9 but deeper than level 7. Makes it easy to use "Find" to zip to a room on my laptop.
Have a Standard Layout - Use the same fonts, emphasis (underline, bold, italics), etc for your rooms. That makes them easier to use at a glance, and speed of reference is speed of actual play.
Strikethrough Preserves Room History - I use strikethrough font to cross out stuff that's not there anymore. This lets me update my rooms without losing details of what was there before.
Putting it all together:
1-5) Elf Wizard's Office. Av/Av Door (DR 2 HP 29, -8). Walls lined with books lettered with gold elvish script. Heavy (300 lb!) wooden desk with floral patterns. Covered with writing stuff (10 lbs, 200 sp value total). Damp.
(3) Orcs (ST 13 DX 12 IQ 9 HT 12 Speed 6 Move 5, DR 2, 4 skull, Dodge 8, Parry 9. Axe-14 2d+1 cut Reach 1, Crossbow-15 1d+4 imp + poison HT-2 or 2 HP. Traits: Bully, Fearlessness 5 (14)) near the door, waiting for intruders.*
Treasure: Orcs 2d cp, 2d sp, 1d-4 gp each. One has bronze earring (30 sp). Books worth 1d x 1d x 100 sp each if taken, average weight 10 lbs each, all in elvish script. Total 120 books but only 22 are still intact due to age and moisture.
I also use strikethrough if rooms change. So if the PCs based the door down, killed the orcs, and ransacked the room, and then some other monsters moved in, I'd do this to it in my file:
1-5) Elf Wizard's Office.
(1) Crazed Troll (ST 20 DX 14 IQ 9 HT 12 HP 20 Will 10 Per 10 FP 12 Speed 7 Move 7 SM+1 Dodge 11 Parry 11(x2) DR 0 Bite-15 2d cutting C grapple on SM+0 Claws(x2)-15 2d+1 cutting C-2 IT(NB,NV) Regen Dark Vision Berserk(BF)-6 Stealth-18 DFM1 p.31)
It preserves the history of it without a bunch of file iterations on my computer. It looks messy but it's surprisingly easy for me in play.
* Too meta of a joke?