Many hydrometers have a set temperature standard of 59F or 60F, your hydrometer should say it, usually on the inset scale or package insert.
This provides an easy overview along with a precalculated table, I believe it is based on a 59F calibration point,
http://www.howtobrew.com/appendices/appendixA.html
And Dreaded Dave's allows you to plug in original gravity data plus the temperature of the must and it adjusts per 60F calibration to calculate potential ACV. Then when ferment is complete, you plug in the final gravity plus temp at that time and it gives you actual ACV, etc. You can also establish actual ACV by plugging in different data combos like if your FG is 1.000 @68F, or 0.998 @67F--so you can get a better idea of where your actual ACV will be if it finishes at a certain SG vs another. It makes a big difference on your ACV if you fail to calibrate.
http://dd26943.com/davesdreaded/tools/convert.htm