wait wait...I got the right formula!!!
for ex.
Growlith vs. Riolu
Growlithe lv. 14, HP=140
Riolu lv. 13. HP=130
Growlith would use say ember on riolu...so here's how it canbe calculated...
Ember's base power = 40
130-40=90
90 would be the remaining HP of Riolu...
simple easy, elementary my friend...put a formula like that on the RP and I say leave it like that...u think so?