Based on warcraft's resistance information table here, which is valid only for lvl 50 spells, I have put together an area chart that shows how your % to resist a spell changes as you gain.

Note when resisting a spell (i.e. Resistance Check) you have 1 of 5 possible outcomes, you either resist 100, 75, 50, 25 or 0% of the spell. Now the chance for each outcome chances according to some formula based on your resist score. But there is nothing here that would indicate that 226 is hugely better than 224 for example.

Limitations
This doesn't tell us the scores for a lvl 63 mob, where presumablly 315 is the max benificial resist. Blizzards choice of 250 for a level 50 player scales exactly with other skills, where each level gains 5 pts/level. So 300 for lvl 60 spell, and 315 for a lvl 63 spell.

Chart


What's next
Presumably, somebody smarter than i can figure out the slopes on those curves and starting with 315 as a max, figure out what the X axis is for the rest of the chart if we were to figure this out for lvl 63 spells (which is what most of us care about).

Does this jive well with other ppls experiences?

I agree, the chart in question is complete for level 50 spells.

The question is what does the chart for level 63 spells look like. It just felt like a really simplisitic assumption that I could just add 65 pts to the top end of the scale and that every thing else would just work, without some math to back it up.

We've had in-depth discussions of resist mechanics on this forum before -- no new ground to break here.