^What Bob said, take 3 measurements and use the narrowest for an inside mount. You can play around with an outside mount, but I think an inside mount looks "cleaner" and shows off the trim.
I have a picture window in my family room and have always had problems with the mini blinds failing, while the other three narrow window blinds in that room held up fine. Center support gave way
I discovered the "we cut to fit free" blinds at Menards LAST!! They get opened and closed every single day, sometimes more than once if the sun gets in the way of Hubby's TV watching. They are more expensive than "blinds in a box", but also had a lifetime warranty, and again I can't stress how much longer they last. Don't know if these still exist as I bought my blinds several years ago, but check them out
Thanks, Chris and Ernie!