For me (enduro) gearing comes down to two questions:
1. How fast will the race be? The faster the race, the bigger the gear.
2. How steady will the race be? (ie, how often will I be making accelerations/jumps?) The steadier the race (or my own expected performance), the bigger the gear.
I use 51x15 (92") as a baseline for local, mid-range points races. For jumpier races (local madisons, points races with very small fields, or especially tired legs) I'll go down to a 90". For steadier races (ie 15km scratch races) I'll use a larger gear - 93" for local events, 95" for national-level events - on the assumption that it'll be fast and require me to keep my speed up and stay sheltered, not jumping or attacking - just following wheels.
The biggest gears I'll use are for local races - short scratch races, tempos, point-a-laps (basically anything under 4km), and for keirins the highest i'll bother going is 96.5" (50x14), though I think I've put a 51 on there before. Doesn't help - me riding a good keirin is about staying in the pocket and anticipating a strong jump, and just rolling it in fully sheltered (Or getting tactically lucky - I won a keirin from the front, once, but it was just a heat which meant that everybody behind me was trying to beat each other, not me).