Throughout my machining career I always aimed for the middle tolerance on anything I made. Not only was that the size things were meant to be, it gave you leeway if there was any play in the machining equipment and allowed for wear on the cutting /forming tools.
Anyway, on topic, have you got a digital vernier to hand? A 12" one should have Jaws big enough to measure the skirt, though it is very difficult to get an accurate reading on a taper. My suggestion, get a metal plate and cut a hole in it to the central tolerance and put the piston in it and measure how far it has dropped in the hole. If the hole is cut to the mid limit and the piston drops in 5mm,or whatever the measurement point should be then the piston will be fine