I don't think the problem is the Boost call amount. It's plain inconsistent.
Also, I don't believe Issei is the one who says "Boost!". It's either the gems, Ddraig or something. That's why the gems in his armor always glow, signifying that his power is being doubled. When Boost is being used, it's shown as [Boost!]. The text is not normally like that if Issei's talking, and I don't remember it sounding anything like his voice in the anime. I also don't remember Issei saying "Boost" himself in the anime or manga. Note: I haven't started watching Hero.
The call amount can't be arbitrary, because in the gauntlet form, we only hear "Boost!" once every ten seconds, but since it can be used without any limits except for stamina in his armored forms, it can be spammed. So, I believe anytime we see [Boost!], it signifies that Issei is boosting his power, and there's really no need to think otherwise.
Same for Divide.
@Robo
That would be calc stacking.