Run sims. Your napkin math isn't really accurate or correct. It uses the right formulas but doesn't show understanding of how they should be applied to the situation at hand. Splitting up the calculations into trying to determine the worth of the weapon when isolating it doesn't make much sense. Compare your stats with weapon 1 vs. your stats with weapon 2, that's the only way to check which weapon is really better.
If you'd want a quick and dirty comparison, try this: "how much % of your total strength do you get from your weapon" vs. "how much % of average weapon damage increase is the change"
Also, when comparing weapon damage compare the average damage, not min/max dmg unless you wanna try a lucky max-dps-parse in which you want highest max dmg and pray for only hitting with max dmg
Napkin math sucks for minuscule stuff, use a sim to compare. It's more accurate and gets the result faster.