Here are the figures from Section 3.7 in the EA. Too many unknowns.
The AML for Porter Canyon is 67 according to Section 3.8 in the EA. Roughly 80% of that allotment coincides with the HMA (see Map 2). The management plan therefore allows only 67 wild horses on a parcel that can support 484 cow/calf pairs (605 × .8).
Yep, 12% of the forage to horses and 88% to livestock—on land set aside for the horses.