An OAT test from Great Plains Laboratory might be more useful. It measures glyoxalate and glycerate as well as oxalate. Together these might give some insight into endogenous oxalate production which may well be making the greatest contribution of all. In addition other markers for things like B6, B1 and biotin dependant metabolism as well as antioxidant status might help to round out a picture of derangement of metabolism which is likely to be associated with endogenous oxalate production.
Here is some of the info they provide about the test.
http://www.greatplainslaboratory.com/home/eng/oxalates.asp
The situation is far more complex than this and many things contribute to oxalate appearing in urine. Here's one paper that looks at some of it .
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2518810/ The contribution from dumping of oxalate from storage is probably the least understood aspect of it.
That figure sounds wildly exaggerated. As the paper above canvases, studies of dietary contribution estimate 10-40% with larger studies, such as that one, tending much more to the lower end. In other words there is certainly some variation caused by diet but it is not great.
You might just decide to try some of the recommendations of the low-oxalate group and see if it makes any difference to the muscle pain - see
http://www.lowoxalate.info/ and the associated Yahoo Group, trying low oxalates. The recommendations include a low-med oxalate diet, probiotics such as VSL-3 to help digest oxalate and encourage dumping of oxalate stores, attention to leaky gut since this can be a major contributer, calcium or magnesium salts before meals to absorb dietary oxalates, gradual increase in B6 consumption, gradual increase in antioxidant intake. If these two latter interventions provoke oxalate dumping this is a very good indication that you have oxalate issues.
The GPL OAT might then give some insight into how serious a problem this could be, but there is no guarantee. There is just too much that is not understood about the whole thing.