Total skill factor is combination of theory and practice factors of the skill.
Example: You have level 4 theory and 81 practice on wrestling skill, it gives total 1.75 x 1.80 where 1.75 is Sage level theory, and 1.80 is Taskamaster practice of the wrestling, so total wrestling factor is 1.75 x 1.80 = 3.15
Maximum skill factor is 2.50 x 2.50 = 6.25 where both theory and practice are maximum 5 and 100.