To try to answer Dave's question: at the last vertical blue I would guesstimate:
15% did nothing at all - just arrive at the platform and rest and then dive.
25% did very little (2 or 3 quick/shallow pull-downs)
The rest did varying amounts of warm up.
Personally I did:
1. "hello ocean" dive, head up, about 10m for 1 min
2. eq exercise (head down, frc, MF at surface, no topping up, falling until the last eq, no riding ears), about 20m 1:30
3. exactly as above but with a little less air, fall until i can't EQ more, about 28 - 29m, 1:30
All the above are about 3 mins in between. 10mins after last dive I am at my OT.