There are no real formulas to calculate correct frame size. I'm currently riding a frame that is "supposed" to be too big for me. I tried out lots of frames that were supposed to be right, and they all felt too small. See Peter White's page on bike fit; I agree with what he say: it's all about your unique body geometry, and your unique preferences and comfort.
If you are comfortable riding your current frame, then it's fine. But if you're feeling cramped and adjustments (stem etc.) don't help - get a new one.