Actually that non spell weapon formula isn't exactly right. It's actually a piecewise function, with amounts over 12 using a different formula than amounts 12 and under...
But anyway, I'm pretty sure the equations that people have found are quite accurate...
As for armor penetration... against a target with little armor, 25% penetration probably makes your attacks 30% stronger. Against a target with very large armor, you could probably get as much as 80% added damage by piercing the armor...
rough estimates based on the equations, I didn't actually calculate it but it looks about right
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