FWIW, that is essentially how you do the math (at least, it’s how WIS does the math). If you go into player roles, create a new player role, and zero out all the non-relevant attributes, leaving only ath, spd, and def, and plug in the numbers TJ proposes, they come out right where he suggests, within a tenth or twentieth of a point. (He’s using Aquino and Guerrera from his Fordham team).
Also FWIW, using 15-18-18, as I do, they come out 81.5 to 74.2.