Look at my handle and guess what I am about to say......average UK SAC is between 25-30, while in warmer clearer climes it is 20-25. My first drysuited dive I did manage to hit 40+ and got through a 12ltr in <19 minutes.
As you would expect I get the p155 taken quite a lot but my regular buddies are pretty good and have been know to help me out when the tank gets a little low.
I always dive a pony now, at least in the UK, as it only takes an unwarranted current or something like that to raise my rate and scupper my plan. Am considering twins so might be interested in the weight of your new set Garf. You up the pool tonight?
Anyway, as Daz said and one of my regular buddies abroad always says you don't get refunds for bringing back air and as long as I get to back on the boat with some left ...
Regards
Mark