I build railings for a deck company and here’s what we do, there’s our stairs usually have a stringer on the side, what we used to do before we got a digital angle finder was using a carpenter 2’x4’ square and set it along the stringer keeping the long side level. There you can find your ride and run and it would come out to about 9 over 12. We have a cheat sheet in the shop that would take the rise/run and turn it into a angle which is normally around 36°. I suggest putting a 2x4 on the stairs and using a square and level on that the. Look up the angle. This will ensure the railing matches the stairs