How to measure for your single return/open on one side
treads.  Measure from sheetrock of one wall to sheetrock on the open side. Or from the skirt or trim board on the wall to the skirt board on the outside of the open wall.  This will give you your throat measurement. Always add at minimun 1/4″ to allow for an out of square wall, or a skirtboard that may get pushed out by a riser.  The throat measurement is from the inside of the ear or return on the open end, to the other end of the tread that will get cut against the wall.
How to measure for a double open tread, or open on both
sides.  Measure the widest point, from outside of sheetrock,  to outside of sheetrock. If you have 3/4″ skirtboards, or will be using them, measure from outside of skirtboard, to outside of skirtboard. If the skirtboard isn’t on, add another 3/4″ per side to the measurement. Measure several places  to find the widest point, if you are needing more than one double return. I even like to add 1/8″ to the actual measurement. It is much easier to spread the skirtboards, than to try to narrow it. The framing won’t move in easily!!

To measure for a plain or wall to wall tread, measure from sheetrock
to sheetrock. Always 1/2″ extra over actual measurement, to allow for an
out of square stair case. If you are using 3/4″ skirtboards, the actual measurement
of the finished tread will be 1 1/2″ less than the sheetrock measurement.