[iamrfixit]

Won't work in the OP's situation due to the taper, but I usually cut aluminum soffit with a circular saw and carbide blade and I cut it right through the full box. It works perfectly, the box keeps the hot little flakes of aluminum from flying around plus helps muffle the noise. Carbide blade cuts it fast, smooth and clean.

First pick up the box vertically and tap it down on one end to get all the pieces tight and even to that end. Then lay it on sawhorses on some scrap lumber for support. The box has one corner that is a glue seam when the box is formed, I use that corner to run the square along and make my marks. Open up the end I didn't tap on the ground to get an accurate measurement for the first cut, then mark and cut each stack of soffit pieces. The F channel and fascia cover give you plenty of room for error but you can still cut very accurate lengths. I make a cut and test fit and install those, then I can adjust cut length if needed.

I was skeptical of this method but after the first cut I was sold. It's super easy. I've seen several other builders and siding contractors that do it the same way.