I agree completely.If you have spent enough time very thoroughly designing/planning your layout, right down to the finite mm. then, for me at least, just as soon as you have cut the first window aperture, then for me at least, the knees stopped knocking!
I approached the first window with more than a little trepidation "oh oh, there's no turning back now...".
As it turned out it when really smoothly and probably less than an hour later I was taking photos of my new window and patting myself on the back. From there I went on to do the toilet cassette door and the second window which of course also all went smoothly.
And with that all behind me, it's all just "house fitting"