Something to understand is that not all stair cases are measured the same. Every step is going to be different.
Please make sure you measure every step.
Measure three points on each step:
LEFT SIDE, CENTER, RIGHT SIDE.
Take the lowest point of that riser as your measurement. Please measure as precise as possible. See Image Below.
(EX: IF the LEFT side is 6 3/4 inches and the RIGHT side is 6 5/8 inches and the CENTER is 6 11/16 inches. The FINAL measurement is 6 5/8 inches)
Measure to the EXACT fraction.
Continue the process for all the steps.
Measure full width of the stair for each step.