Measuring for Vertical Blinds:
Please note: You should always use a metal tape measureRECESS
GIve us the tight measurement and we will make the allowances

Width = A - B
Measure at the top along the line of the Headrail.
Drop = C - D
Measure at the sides and the middle and give the shortest drop
OUTSIDE RECESS
This is the actual size you want the blind to be made

Width = E - F
Overlap the recess by approximately 25mm to 50mm either side to suite your requirements IE: allowing for any obstructions.
Drop = G - H
Measure 100mm approximately above the recess to the point where you want the Blind to finish.