Stair Calculator - Layout Stair Stringer Headroom Rise Run Baluster
He's right about the stringers may come out differant.
One thing to always check is how level the floor is where the bottoms going to sit. Out of level and the stringers could be cut differant or a form made and self leveling compound can be pored into the form to get it level.
Any way to make those stairs wider?
Once the treads are cut and test fitted your going to have to cut a space in the bottom of them to fit a 2 X 4 or 2 X 6. It will keep the stringers from moving and give you a place to Tap Con screw the stringers to the floor.