resistances

what would a fair price be for these ? :) im really new to rare boots

If I have 15.1% resistance to Lightning and I add a Diamond 💎 to a ring which adds 6% to all resistances what will take my lightning resist to.....? I thought they were additive so I'd increase it to 21%. Nope. So I thought multiplicative which (if I'm not mistaken) means 6% of 15% added to...