In article . com,
writes
Measure the minimum distance from top of masonry cill to bottom of
lintel. This is the size of the actual opening in the wall. Deduct a
few mm to allow for irregularities. Order the height of the wooden
window accordingly.

In these parts (scotland), this sort of window is specified by the 'stone
opening' size, the wooden cill will rest on the stone cill but top, bottom &
sides will overlap with the stonework but sit inside. In this case there is no
need to deduct from sizes. Typically 10-15mm of the frame will be visible
at sides & top when the window is fitted.

Quoting the stone opening size when ordering puts all the responsibility on
the manufacturing joiner to get the sizes right.

Glazier?, misnomer or a bit of a funny setup.
--
fred