The first time I built a freestanding bookshelf, I put a lot of effort into making the feet level and the back straight and at a right angle to the feet. Once I put it up against the wall I'd built it for, I realized I'd solved completely different problem than the problem I really had. I needed crooked bookshelf, since the wall was totally tilted.
In the end I screwed some wall shelves in and called it good enough.
In the end I screwed some wall shelves in and called it good enough.