Measuring for roman blinds outside recess

[whichlower48]

Roman blinds outside recess, are they supposed to sit right on the window sill when they're down or drop lower than the sill? I'm adding on 3 inches on the left and right and 3 inches on the top but I'm unsure whether to add 3 inches to bottom or measure so they sit exactly on the sill.

This is totally your choice. Would you rather they end at the sill or hang lower?

Are you planning on using a covered wooden baton or a roman blind cassette?

If, whatever you do use, projects the blind to at least the depth of the sill (away from the wall) you can have the blind lower than the sill.

There is a fabulous tutorial for making roman blinds the professional way - with no visible stitching lines on the face fabric...
https://www.sew-helpful.com/roman-blind-lined-index.php