The Browning Hi-Power magazine case has a 12.5° forward angle with respect to the floor plates:

The natural axes lie parallel and perpendicular to the case axis, which means dimensions parallel and perpendicular to the floor plates (horizontal & vertical, respectively) require a bit of trigonometry. This doodle sketches some of the key values, not all of which are hereby asserted to be correct:

Name the variables:

- Slant angle α
- Height H along magazine axis
- Length L perpendicular to H

Components of H:

- vertical = H cos α
- horizontal = H sin α

Components of L:

- vertical = L sin α
- horizontal = L cos α

Extreme point of the tilt at the edge, relative to center point on axis:

- vertical = (L/2) sin α
- horizontal= (L/2) cos α

Projection of top parallel to axis onto horizontal:

- L / cos α

I suppose one could set up functions for all that, but I tend to just hammer out the trig where it’s needed.