Just measure twice and cut once.
If you're cladding the stairs in timber for the treads, you need to work it all out in section to scale first, and the depth floor finish at the top and bottom of the stairs must be taken into account.
This is true to a degree for all stairs, but mroe so with a concrete stairs and flor upstairs, particularly if you're thinking of 25mm treads and perhaps 50mm stone on the hallway and only a carpet on 1st floor - you need to work it out properly.
Trip hazards at the top of stairs can be lethal, and at the bottom can lead to stumbles - a real risk as you get older.
ONQ