Alternator Sizing - Amp requirements
Somewhere buried in the technical books you and I can never find are RPM vs output curves for alternators. You might check with some of the better aftermarket outfits who sell that sort of thing
If you have not, be certain you deal with the "bulkhead connector/ ammeter bypass" if you have not done so
Also, it is my personal preference AGAINST "one wire" set ups. They make it more difficult for battery disconnect situations, and they require a MUCH larger charge wire because the sensing is on that wire
You are right tho, adding up and estimating draw is the "right" way