Roman Blinds hang over part of the window, even when raised, so are often mounted outside the recess
For a great look mount the Roman Blinds so that the overhang is around 80 above the window recess
By allowing it to drop slightly below the recess you can not see any issues if the top of your recess is not level with the sill
Ideally you also want at least 50mm additional each side or the same width of your sill depending on the circumstances
Ask us questions at advice@directblindsandcurtains.co.uk or @DBlindsCurtains, include a photo they say a 1000 words