For fitting inside the window recess the size entered should be taken from the top of the window. The cassette will always be made to the top width.
There is a product tolerance of +/-20mm in the width, this means the bottom width can be 20mm wider or narrower at the bottom. and +/- 6mm in the drop
For an surface fix the cassette will always be made to the exact width sizes given
There is a product tolerance of +/-5mm in the width, and +/- 3mm in the drop