I have always just used my 6' ladder as the hight off the floor, I measure from the top of the ladder to the ceiling box, then deduct for the canopy and the fixture and that is my chain length.
Too low and people will hit their head when getting up from the table, too high and just don't look right, I make a couple extra hooks and if construction is still going on I will hang it all the way up out of the way, and when the builder does the final cleaning they will lower then back down.