What is the width deduction, and who applies it?

The width deduction is the small amount a factory subtracts from your inside-mount width so the finished blind clears the frame and operates freely — commonly 1/4" to 1/2", though it varies by seller and product. The detail that trips people up is who does the subtracting: most sellers deduct for you (you give the exact opening size and they take it off), but a few expect you to deduct yourself, and getting that backwards double-deducts and leaves a gappy blind. This is the most consequential number in a custom order. Confirm against your seller’s measuring guide whether to give the exact opening or a pre-deducted size — their rule wins, not a general one — because custom blinds are generally not returnable for a mismeasure.