I'll agree that the "ideal" is equal intake and exhaust fan capacities. Well, almost - actually I prefer slightly more intake capacity so that excess air can leak OUT of the case, not in, because inflowing air carries dust. HOWEVER in practice you cannot figure out the air balance from fan capacity specs because ALL of your INTAKE fans ought to have dust filters on them. The specs for the fan cannot take into account the reduction in air flow caused by the filters. So what you really end up doing is setting up fans, the using some tracer technique (some have suggested the smoke coming off an incense stick) to test the air flow direction at case leakage points. Adjust your fans if necessary.