For niche mounting, measure the niche in 3 places minimum (at the top, in the middle and at the bottom). Choose the smallest measurement and deduct another 5 millimetre. The reason for this is to make sure the roman blind fits nicely in the niche and that the fabric does not scrape against the sides and wear out.
If you mount the roman blind outside the window on the wall, one option is to measure the blind in line with the window frames. If you use a blackout roman blind however, it’s a good idea to let the blind be a bit wider than the actual window to minimize the light coming in on the sides.