I never did this before so can't help much but all I can say is..... Measure first, then measure again, then just in case measure again and then measure one more time and then for the He** of it measure again and then just to be REALLY REALLY SURE, measure for the last time.... Did I mention you should measure?
Hold tight and you will get some responses more helpful than mine, lots of old timers on here have done this and can help you.