No, it's definitely not that simple.

It turns out you can't actually detect enemy Defence or Offence Rating with Loremaster, it just gives you the Chance to Hit directly. Perhaps more useful in combat, but less useful for theorycrafting. You can, however, determine the chance to hit with friendly fire on fellow party members, so you can collect data that way.

Here are some numbers, Offence Rating first:
151 v 95 - 90%
151 v 105 - 82%
151 v 165 - 27%
151 v 170 - 22%
157 v 165 - 33%
157 v 95 - 98%
157 v 90 - 100%
157 v 170 - 28%
175 v 105 - 94%
175 v 95 - 100%
154 v 165 - 32%
154 v 90 - 99%
154 v 105 - 88%
154 v 170 - 28%
181 v 105 - 96%
181 v 95 - 100%
163 v 165 - 34%
145 v 140 - 43%