Woketman you are right, there are a lot of different factors. These make it really complicated for full scale layouts that have to be approved by authorities. But laying out a spar for worst case scenario (no support from fuse, loads induced at FCL and WS/4) will get you in the ballpark plus safety. I've taken average strength of market-usual HT carbon and placed them in the standard ME formulas for 2-dimensional loading. This gets you a good impression of where you are, and a test on a crashed wing (not damaged on critical points) has shown that using this method you will be within 15% of the actual burst load....close enough for 99% of all modeling needs.
Also, using average strengths published, you will probably land somewhere near the stuff used by most companies...