I would like to share with you my way of cutting 45° for frame.
Instead of messing-up with protractors, I used big dedicated plastic triangle.
For a 45° frame to be square and the angles to fit, I made 3 things:
1. I’m cutting the frame members (2 and 2) to the same “final length” (see Note).
2. To overcome small deviations from 45°, I'm cutting the 45° on the left and right of the blade, so each corner will fit horizontally and vertically (If the blade is not exactly at 90°).
3. To be sure that; when I cut the 45° the length of the members will not change I made kind of “stop” or “limiter” as you will see in the pictures.
Note: Because the vertex of the triangle is at the center of the kerf, each cut will reduce the length of the frame member by half a kerf, and becase we are cutting both sides of each member, each member length will be shortened by one kerf length.
To make it clear; if you are using 1/8” kerf blade and you are planing a 20” x 20” frame, cut your members to “final length” of; 20-1/8”, the 1/8” will be removed during the 45° cutting.