Roman Blinds
When Measuring for Roman Blinds I would normally add 150mm to the Recess drop, so the fabric is just below the the Recess when the Blind is raised.
For the sides, if there is room I would add 50mm, to the Recess Width, on each side, or if its a similar size, the width of the sill.
When you fit the blind it can then be aligned to the sill, that looks great.
These Measurements would be ordered “Actual Size”.
Roller Blinds
With Roller Blinds you would add 100mm to the Recess drop and again 50mm, to the Recess Width, on each side as before. However remember to order the Roller “Fabric Size”.
The Brackets and the controls of the Roller will protrude by around 18mm past the fabric on each side, so ensure you have enough clearance.