I dont think it's a straight multiplication or division however; it appears to be an addition or subtraction of the percentage instead (think current value minus 0.15max value, min value = 1). At least it seemed that way in the heat of the moment... I could very well be mistaken on that point.
If that is the case, it would really be the equivelent of loosing the regen. For example, let's only consider the current value for, say, available MP (not total). Let's graph that value with respect to time. One would expect to see a linear curve, increasing with a positive slope that is equal to the mana per second regen value of the demigod until the max mana were reached, at which point the slope would become 0.
However, the loss of the MP flag will cause a discontinuity in that curve of currently available mana, which could make the value of currently available MP *less* than the original starting value. If we were to calculate the *effective* regen during that time, say from time t=0 to time t=1 second after loosing control of MP flag, the mana per second regen would be effectively *negative*, as you would have (mana final - mana initial)/(elapsed time) as a negative value.
On a side note I noticed this while playing a game as oak recently. I was left alone on the MP side of cataract for a minute or 2 after driving off an opponent, but I was pretty banged up. Monks and divine justice were slowly healing me back up, but on the other side of the map the HP flag was getting traded back and forth a few times between my team and our opponents. It seemed like I basically couldn't heal, as the current HP total kept on getting banged up as a result of the HP flag trading. As I said, I could be mistaken in how the math gets done... Haven't had the opportunity to verify today yet.