I'm about to do some research with Nash tables, now I want to create an equity heatmap for the starting hands.
Is there any canonical way to calculate their relative rank? Perhaps a conventional source?
No. There is no "generic" way of calculating the equity of a given confrontation except for calculating it directly. For example, bigger pairs have a 4:1-ish advantage over smaller pairs, but the odds are not exactly the same in AA-KK than in AA-77. As a rule of thumb, with round numbers:
Bigger pair vs two lower cards (paired or not): 80%
A pocket pair vs two unpaired overcards: 50%
A pocket pair vs two cards (one higher, one lower or equal): 65%
A pair dominating two unpaired cards (like 99 vs 94): 90%
Two unpaired high cards vs two unpaired low cards: 60%
Dominating unpaired hand (like AK vs AJ): 70%