I've been doing the dive planning for my AN/DP checkout dives. My instructor told me to plan 2 dives, on successive days. Both to 120' for 35 minutes. I'll be using double HP120s and an AL40 for deco gas.
After going through this exercise, I am very glad I did not get doubles that are any smaller.
I ended up going back to my instructor and proposing we shorten the dives to 30 minutes because, essentially, I was gas limited by the 120s. The dives are long/deep enough that my planning showed I would need well more than half the gas in my deco bottle. That, in turn, means I have to accommodate a lost deco gas scenario in my planning - i.e. have enough back gas to do all my deco on that, if necessary.
If I were diving, for example, double 100s, I would have needed to shorten the dive even more. I think 30 minutes at 120' isn't really that long, so I am definitely glad I don't need to shorten the dive any further.
Of course, if everyone took an AL80 for deco gas, instead of a 40, it wouldn't be an issue. Then I could probably do the 35 minute dive on double 100s. But I don't really want to step up to carrying a deco bottle that big just yet.
And, yes, my planning is using very conservative numbers for consumption. These will be my first real deco dives, so I don't mind being extra conservative in my planning and having 5 minutes less bottom time as a result.
Just another thought on the size of doubles...
An ali 80 really isn't a big deal in the water. Out of the water, it's more cumbersome, but in the water, i hardly notice the difference between it and my 7l (roughly an ali 50).