In relatively recent times, the description of the Brawny perk was changed to state that the fatigue penalty would be reduced by 30% (down from the previous 35%). In practice, however, it’s still always closer to 35%. For example, with a total body+head armour cost of -13, you’d expect the -13 penalty to be reduced by 4 (30% of 13 = 3.9) or by 3 (if rounded down), but it’s always reduced by 5 instead (35% of 13 = 4.55).