Meybe, made bonus amount bars dependent on Alloying and Sparks & Embers ?
Example
Bonus amount bars formula = (Alloying/1000) * (Sparks & Embers Value - 100)/100
Sparks & Embers Value = 1000
Base Set Alloying = 1000
will give bonus +9 bars
IF number bonus is huge it's possible manipulation two way
formula = (Alloying/2000) * (Sparks & Embers Value - 100)/100
give +4 / +5
or
formula = (Alloying/1000) * (Sparks & Embers Value - 100)/200
give same +4 / +5
but two other "mean"