Difficulty rises as damage done with a kick to that zone rises.
In order of difficulty and damage, lowest first. shins, feet, knees, chest, stomach, face, back of the head
Test bonus: 255. Mixed foes, 1 hand free, attitude wimp.
1 is a prepared kick. + is a prepare with flavor text (strong prep). 0 is prepare with flavor text (weak prep). x is a failed prep. result column format is strong prep:prep:weak prep:fail
Feet: 1111++++x111111 = 4:10:0:1 (TM on regular success, no bonus change).
Shins: +++++1++1+++1++ = 12:3:0:0.
Knees: 11+x1111+1x+x00 = 3:7:2:3
Stomach: 011+x+x11+111+1 = 4:8:1:2
Chest: 1++1+1x+++111++ = 8:6:0:1
Face: xxxxxxxxxxxxxx1 = 0:1:0:14
Back of the Head: xxxxxxxxxxxxxxx = 0:0:0:15
Difficulty shown by 'kick compare' seems to pan out in the real world. Knee results here somewhat anomalous but likely owing to small sample
There appears to be a significant step between stomach and face. 255 bonus is barely enough for occasional face kicks and never preps back of the head, yet is moderately reliable vs stomach