Yes, exactly. It could be easy to short draw a heavier bow, and you want to find your true draw length. There are also some decent wingspan methods that usually get you pretty close.

Here is a quick way to measure it, it will be close, but not exact, some people say that they have longer DL then below, but I'm pretty darn close:

Place a yard stick or tape measure in the center of your chest. Reach both hands forward and look where your finger tips rest on the measuring device, don't stretch. Do this about three times or more to get the average.

Or this one:
http://www.pabucks.com/bow_draw_length.html