Here are the results (0 seems weird and I just thought... Maybe the damage is actually 150.x and the game is rounding it up for the pip boy screen but only applying 150. If so my numbers will be off by about 0.7% on all accounts)
Physical Resist = Damage Reduction %
0=0.7
1-20=1.3
25=3.9
30=10.6
35=15.2
40=20
55=27.8
60=30.5
65=32.5
85=38.4
95=41.1
210=58.9
595=75.5
1195=85.4
So it appears that any armour up to 20 is basically useless. After that it grows quite quickly to 40 or 50, then slowly starts tapering off in effectiveness per additional point.
You still see decent returns up to 100ish but then after that you start getting into power armour territory and you need lots to get the same results (of course since the higher your resist percent, the more effective HP you get per point of resist, it's hard to tell without doing math where the benefit falloff really starts).