Measure the right window blind width and drop for inside or outside mount.
Blinds are ordered to the blind size, not the window size, and the two differ depending on how the blind mounts. An inside mount sits within the recess, so the supplier needs the opening width minus about half an inch of fitting clearance and the height minus a quarter inch, otherwise the slats bind on the reveal. An outside mount covers the frame, so you add overlap on each side — 1.5 to 2 inches is enough to kill the light gap — plus roughly 3 inches above the opening for the headrail and brackets. Measure the recess in three places and order to the narrowest: windows are rarely square, and a blind cut to the widest measurement will not drop.
Blinds Size
Inside mount: width = opening - 0.5 in, drop = opening - 0.25 in. Outside mount: width = opening + 2 x overlap, drop = opening + overlap + 3 in headrail
For an inside mount, always down to the nearest eighth of an inch and to the narrowest of your three readings. For an outside mount, rounding up only adds coverage, so it is safe.
Because openings are rarely square. A recess that is 36 in at the top and 35.5 in at the sill will jam a blind ordered at 35.5 in net width partway down.
When the recess is under about 2 inches deep, when the window is out of square, or when you want maximum blackout — the overlap removes the light leak an inside mount always has.