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.
but, unless the beer spent some time at higher than 70 that mistake would result in overcarbing as the calculator would recommend more sugar given a higher temp.
?? I think your math is upside down. If you should calculate for 70 degrees, but you instead calculate for 40 degrees, you'll wind up adding less sugar than needed - which is exactly what you said, but not.
right, but the OP said he calculated for the temp the bottles would condition at (70*) I assumed that he fermented cooler than that so if he calced for 70* and the beer never went warmer than say 65* it would be slightly too much sugar. probably fairly negligiable(no spell check at work ) either way though