The thing is, because the amount of fentanyl is so small in each pill (akin to perhaps 20 grains of fine table salt), along with the lack of quality control, I wouldn't rely on the fentanyl being evenly distributed through the pill (like one would expect from a professionally made pharmaceutical pill). This means 1/4 of a pill might be much stronger than a quarter from the very same pill. I've heard of people having some nasty surprises as a result of this. If it were me, I'd grind them all into dust and mix thoroughly and then dose by volume (or even better, by weight).
When I was using fentanyl I never came close to overdosing, but I had a massive tolerance (I used to shoot the 100ug/hr fentanyl patches in a single giant shot), and given the pharmaceutical origin, I always knew how much I was taking. Sometimes I wonder what would have happened to me if street fentanyl had been common a decade ago when I was doing stuff like that. I may not have survived. I mean shit, fentanyl is supposedly either one of the, or, the leading cause of american adults 18-45 years old.
In any case, I know people don't like it when I say it, but if you're down to doing dirty 30s you might consider just going suboxone or methadone (the latter worked for me, for the most part).