Did you test with and without Fel Armor, does it stack with the +healing you get from the armor? Judgeing by your numbers it only got the 40% increase instead of a 60% (health only) or 20% increase then 40% increase again health only and that represents Fel Armor and then the stone. However like you said the mad alch pot is random between two set numbers, so would this work on say a dreamless sleep pot? that should give you an exact amount of health/mana so it might be easier to calculate.

Thanks for any responses!