We usually tell our customers to measure the width of the window opening at the top of the recess. When the window blind is manufactured we will make the necessary allowances to ensure that the blind fits correctly.
Venetian blind measured between half tiled recess
Venetian blind allowing for tiles at bottom of recess
This advice is correct when measuring vertical or roller blinds because the fabric is not quite as wide as the recess. However venetian blinds are as wide at the top as they are at the bottom. Therefore an allowance has to be made to provide clearance for the tiles if the recess is partially tiled at the bottom.
You must measure venetian blinds at the bottom when the recess is partially tiled.
When measuring partially tiled recesses, measure tightly between the tiles and take off 15mm. If you don’t, the blind will clash with the tiles when the blind is lowered.