You could use the square and hand held saw, but that means marking where the cut is needed, then measuring back along the board to allow for the saw base. Easier to by a drop saw, that had a wide enough cut, or if you can get away with it a sliding compound saw for even wider cuts.
if it was easier and quicker to do with a drop saw I would do it that way, it would save me time and money...but it's not, so I don't use one on fascia, or cutting things like verandah decking boards either. There is no need to mark back from the cut line, just start the saw with the square holding it slightly long away from the line, then slide the square slowly back until the blade hits the right spot to cut it. When you've done a job for 25 years you learn what works best to save time, your way will work, just takes abit longer is all. When you get up on the scaffold and find it's a touch long or the cut is out abit, have fun climbing back down and taking it to your drop saw...but if you have a square and power saw on the scaf with you, just adjust the cut and keep going.