I also found out that putting the top screw in the brace on the wall first before drilling the second hole is a lot easier, it makes it less difficult to keep it level. (But in the picture I had drilled both of the holes, it caused a lot of problems for me because they weren't level even though I tried, it was just harder overall.)
Drill the second hole after re-leveling and put the last screw in.
I found that using just two screws, on the top and bottom holes, works just as well, and it saves on screws. (I only put two on the others bookshelves I made, except on this bookshelf only one screw went through the stud and I put the other through drywall and added the extra, just in case)