Windows are measured and quoted by the united inch, what is a united inch you say?
United inch is the width plus the height, Example say the width of the window is 32 inches and the height of the window 54 inches if you add these together 32 plus 54 you would get a total of 86 united inches
Measure the windows from the inside of the trim of the interior window.
If there is no window trim, measure from the inside of the plaster to plaster.
As long as your measurements are to the whole next inch, we can validate your quote.