Magica.X+SpellDmg.Y = Heavyattack per second
x= 0.12
y= 1.2686
LightAttack HoT=Heavyattack*2.3924
Round the answer to integer, and thats it.
Example 1
Magica 36275 and SpeelDMg 2118
Heavy = 7040
Light = 16840
Example 2
Magica 36275 and SpeelDMg 2541
Heavy = 7577
Light = 18126
Example 3
Magica 34142 and SpeelDMg 2118
Heavy = 6784
Light = 16230
Try it and let me know if you get something different, I also tryed mod Champion Points like Staff Dmg , elemental, etc but no changes to results, only Magica and Spell damage seems to work to me.