I use the property blue prints to calculate my measurements; if you are talking about knowing the right information to advertise. If for other reasons, then I may be able to give you a method which will be better. They have lasers for measuring and that is very accurate as well.
For painting, if you are using paint from the same location, you can have them come out do the measurements and at the same time get samples for your walls and trims, so that when paint is being ordered they will know up front your color scheme as well as how much they should be making for you. You can always check their measurements against what you have. I would also check your historical records to see if there is a difference as well.
I hope this assisted somewhat!