Just give the width and height of the recess exactly. We will
allow the working tolerances N.B. It is advisable to measure
the width and drop in three places and quote the minimum width
and mimimum drop.
Normally this measurement is used for outside the window. Decide
how much you wish to overlap the recess at each side (suggested
overlap is 50mm each side) and at the top so that the blind
is fixed away from the edge of the reveal. Some blinds may need
to clear the window, if fitting wooden blinds add 50mm (2")
plus 12mm (0.5") for every foot of blind depth.