Rule of thumb estimations are exactly that. If you build a house like that it will be a bit janky
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It likely also depends on the class. Some such as knights have less attack than rangers/warriors/monks. 100 attack (I’m assuming strength is already capped) in theory should do more for them than the others. Still, a 10-15% spread feels right. I’ve parsed a lot with my non-primal Pally next to another with similar gear and they are frequently about that much higher. The ranger is more wonky due to getting bow crits; on a short fight a few 400dmg hits with the BFG certainly pushes up the average. In my one test though I’m certainly a believer.
If you think about it, 5-10% is a solid upgrade especially if you’re 60. Most ToV upgrades going from near BiS to BiS is about that. For a ranger it’s like Kflame to Baton of Flame. For a SK going from a Reaver to a Rocksmasher is about 13%. An Eyepatch of Plunder is 20% haste but at 100% haste it reduces your delay by half; so really it’s about a 10% increase while soloing.
Not to put my thumb on the scale of what you should do (I’m tired of fighting) but this is a slow and relentless math game. NPC’s scale much better than PC’s. Ragefire gloves and turtle belts are silly on paper but it’s important not to get complacent with stats. Carry a bunch of things to swap out, whether it’s mana gear, str-dps gear, and tanking stuff. You can always destroy muffins if you win a /random in a dungeon.