I would be a little careful with this approach, as the preparative procedure described is likely (if not very very carefully controlled) to result in SR vesicle damage (it is not in the form of individual vesicles in the myocardium, after all), and indeed calcium efflux from intact vesicles, so significant calcium is likely to be lost from your SR preparation. You may want to try some control experiments where you empty the SR first with say caffeine before isolating the vesicles. That having been said, most colorimetric methods should be OK in terms of sensitivity, but can be problematic for solutions containing suspended materials (e.g. proteins), which can affect reliability.

Generally speaking it might be better to consider doing the measurements in intact live cells using a fluorescent dye, most usually Fluo 5N (see Molecular Probes catalog), though precise calibration can be a problem.