Good question. I'm no expert on resists as I mainly play melees and hybrids, but I'll throw some info on the table and people can correct me where I'm wrong.
The first thing to know is that the maximum resist cap/resist chance is something like 95%. This means if a resistable magic-based spell is cast on you and your magic resistance is maxed at 255, there's still a 5% chance it will land regardless of the caster's level. If we're scaling linearly assuming 255 is a hard cap, then each point of resistance makes you .4% more likely to resist a resistable spell of that element, meaning 7 points would give you a whopping 2.8% more resistance power. Of course it rarely actually scales linearly because there are always so many different variables in play: your level, the caster's level, the resist check of the spell (if it's a resistable spell), and your resistance. If you're the same level as the mob/player casting on you, then I'm assuming it would scale linearly and each point of resistance would be a steady .4% increase before other factors come into play.
So obviously there are bottleneck points when it comes to resists and levels - I'm assuming the 3-level difference mark is a big one, then probably another at 10, and so forth. All resistable spells have what's called a resist check, ranging from 0 to 300 - I'm guessing that resist checks like those of the Lure line nukes literally just shave that amount off the target's resistance when it calculates a hit/resist, but since Lures are still resistable there's always a 5% chance it will be resisted. Most spells have a resist check of 0, and you can also partially resist most spells which docks some amount off the damage or duration of the spell that lands on you.
Similar to levels, there are also bottlenecks at certain points in the 0-255 resistance scale, although I'm not sure what the hard data is or how the mechanics work for this. 120 MR unbuffed is the entry-level resistance for raiding, and competitive raiding guilds usually require 150 MR minimum, so I'm assuming that when dealing with level 60+ raid mobs, these are points at which the scale slopes upwards. The same can probably be said for 200, and there's speculation that 255 is only a "soft" cap - that if you get to 200 unbuffed, and then add another 100 from buffs, you'll have more resistance power than the 255 displayed in your UI. I've yet to see any substantive evidence to support this theory.
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