Mark across the unopened box of soffit material with a square and saw right through the whole box at once!
The box protects the pieces from getting scratched, controls much of the noise and also helps to contain some of the tiny bits of aluminum that inevitably fly around when cutting metal. I usually open the end of the box so I can be sure the ends of the panels are aligned and I can get an accurate measurement. I just use my regular circular saw with a regular carbide framing blade. Cuts like butter and leaves a real clean edge. Might seem strange but it does not act much different than cross cutting a larger board like a 2x12.
Almost every builder I have seen does it the same way. It works very well and is still very accurate. The best part is you get a whole stack of cut pieces with only one cut. Verify your measurements carefully though because you can get a whole stack of mistakes with one cut also!!
Works best with full boxes but I have also cut partial boxes this way without issue.