Is that the TOTAL amoung of mag & zinc you're getting from the multi? Some have you take 2 tabs/day so you're getting double what it states per tab. Double-check that.
Chances are you're not deficient enough in those minerals for ZMA to benefit you much. Before trying ZMA I was taking my multi plus 1-2 tabs of a calcium/mag/zinc supplement, so I seriously doubt I was deficient...that's probably the reason I didn't notice any gains from ZMA. It just helped me sleep better, which to some people is worth it anyway.
If you're really serious about finding out, have your blood tested and check the results. My opinion on ZMA is that it probably works, but ONLY if you're deficient in those minerals to begin with.