Most common is the 7/11 rule. Once you determine how many risers you need by adding one or subtracting one to keep within the rise code of 125mm to 200mm you can then use ration and proportion to determine the tread depth. Rise/run = Rise/run but replace one side with 7/11. Example 7/11 = 150/X or 11x150÷7=tread
Calculation is always done from finish floor to finish floor. So you need to know the material thicknesses to calculate an even rise. But when building the stringers don't forget to add the flooring thickness to the bottom of the first Stringer (bottom riser) so when you place the finish floor after the rises all match. Then the top Stringer (top riser) would need to subtract the top floor thickness so that after the finished floor is added the risers are all the same.
9'= 108" ÷ 8" = 13.5 (to stay within code you add or subtract a riser) in this case you make it 14. If you chose 13 the rise would be 8 5/16. That would be too high of a rise and against code.
