So like many legendary proficiency doesn't feel legendary compared to Untrained/Trained to me, so i played with some math and feel personally that if Proficiency equaled 1/2 level rounded up plus U -4 / T +0 / E +2 / M +4 / L +6, it would show this better.
I did some math on it figuring out percentage chances of hit etc. first column is proficiency of the weapon attack and the rows are proficiency of the armor the bold is when the proficiency match.
So this math is based on a lvl 20 character with Str/Dex 10 (+0), a +5 Longsword, and +5 Full plate.
Prof | Atk || Def | U | T | E | M | L |
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U | 11 || 27 |16+ 25%|20+ 5%|22+ -5%|24+ -15%|26+ -25%|AC
U | 11 || 23 |12+ 45%|16+ 25%|18+ 15%|20+ 5%|22+ -5%|TAC
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T | 15 || 31 |12+ 45%|16+ 25%|18+ 15%|20+ 5%|22+ -5%|AC
T | 15 || 27 | 8+ 65%|12+ 45%|14+ 35%|16+ 25%|18+ 15%|TAC
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E | 17 || 33 |10+ 55%|14+ 35%|16+ 25%|18+ 15%|20+ 5%|AC
E | 17 || 29 | 6+ 75%|10+ 55%|12+ 45%|14+ 35%|16+ 25%|TAC
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M | 19 || 35 | 8+ 65%|12+ 45%|14+ 35%|16+ 25%|18+ 15%|AC
M | 19 || 31 | 4+ 85%| 8+ 65%|10+ 55%|12+ 45%|14+ 35%|TAC
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L | 21 || 37 | 6+ 75%|10+ 55%|12+ 45%|14+ 35%|16+ 25%|AC
L | 21 || 33 | 2+ 95%| 6+ 75%| 8+ 65%|10+ 55%|12+ 45%|TAC
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So common modifiers to this are:
Flanked | AC -2 | +10%
Light Shield | AC +1 | -5%
Heavy Shield | AC +2 | -10%
Dex Mod +1 | AC +1 | -5%
Str Mod +1 | Hit +1 | +5%
Str Mod +2 | Hit +2 | +10%
Str Mod +3 | Hit +3 | +15%
Str Mod +4 | Hit +4 | +20%
Str Mod +5 | Hit +5 | +25%
Str Mod +6 | Hit +6 | +30%
Str Mod +7 | Hit +7 | +35%
This creates a larger gap between the legendary attacks and defenses compared to trained, and flanked now essentially drops you 1 grade in proficiency instead of 2 grades. According to the math i'v played with a character gains 5% chance to succeed for each proficiency rank gained.
EG. L 3+20+5(28) vs L 10+3+20+11(44) = 16+ to hit or 25% chance to succeed.
OR. L 3+20+5(28) vs M 10+2+20+11(43) = 15+ to hit or 30% chance to succeed.
OR. L 3+20+5(28) vs E 10+1+20+11(42) = 14+ to hit or 35% chance to succeed.
OR. L 3+20+5(28) vs T 10+0+20+11(41) = 13+ to hit or 40% chance to succeed.
OR. L 3+20+5(28) vs U 10+-4+20+11(37) = 9+ to hit or 60% chance to succeed.
Thoughts for anyone that read through all that and can follow my erratic thought paths?