Oh I got confused for a bit b/c you were asking about hydrostatic weighing for a body fat percentage but wanting to know your RMR. It will vary from person to person. Getting an exact figure generally requires a person to do a metabolic study where you are put in a room and they measure gases, energy expenditures, etc..basically, as usual, what JohnP said.
Do you really NEED to know an EXACT figure? It might be nice but it's very costly and not really necessary. You could always eat at one solid calorie allotment for a week or two and tell by reversing the math approx how many calories you tend to burn in a normal day. Some of the online calculators have been pretty accurate in my experience in calculating what I "generally" burn. Really, it all depends on IF spending that money will make or break your weight loss. I have to say though, even if you were to get THAT number, you can't be guaranteed to lose weight predictably. Loss is rarely the same week to week for any person. It's really a matter of staying in the game.