If you're using a priming sugar calculator like that, make sure that the temperature you use to lookup is the highest temperature the beer ever experienced after fermentation. So if a beer fermented at 68, but then warmed up to 75 afterwards, and now it's cold conditioning at 40 - lookup the priming value for 75 degrees because that high temperature will drive out much of the dissolved CO2 and it won't go back when the beer cools. Not sure if this applies to you.
Hmmm... I thought it was the temp that the beer would be bottle conditioned at.
So, do you think this batch is a dumper or can it be saved?