If you have Act 1 Merc with 300 weapon damage and her stats is 500 str and 500 dex, and you give her +300 extra attribute either it is dex or str it goes like this:
If it is extra Dex 300 = total 800 Dex = Damage Bonus 160% (all bow have 20/100 dex bonus damage) + 500 EWD from 500 str.
Damage = 300 + (200%ED + 160% dex bonus damage) + 500% EWD
= 1380 + 500% EWD
= 8280 Damage plus 125 Energy Factor and 500 life regeneration from 500 Str
If it is extra Str 300 = 800 Str = EWD 800% + Damage Bonus 100% ((500 dex * 20)/100).
Damage = 300 + (200%ED + 100% dex bonus damage) + 800% EWD
= 1200 + 800% EWD
= 10800 Damage plus 200 Energy Factor and 800 life regeneration from 800 Str
Same goes for Act 2 merc with the different is all Spear and Scythe have 30/100 dex bonus damage.
Act 3 merc have 200 energy + 20 energy per level which is 200 + (20*119) = 2580 Energy at level 120, he also have 250 energy factor + 5 per level which is 250 + 595 = 845 EF at level 120
According to this graph
https://docs.median-xl.com/doc/concepts/energyfactor, with 2580 Energy and 845 EF it is about 400% spell damage in level 120 without item, and if he have total 800 str after equipment that means an extra 200 EF which is 1045 EF but according said graph, the extra spell damage is still in about 400%-450% range, same thing happen if you give +300 Energy, 2880 Energy + 845 EF still in the 400%-450% range, you will need 3000 Energy and 1000 EF to get 450% spell damage.