[Herb Stoops]

The ‘customer’ was especially taken by them but to me they were a design nuisance. Here’s why - anything off a 90-degree cut widens that mitered surface’s width, so each of those 45-degree radius cuts added half again as much material to mate up as a simple square corner would have. Lining up the interior surfaces left a good 1/16” of material sticking past the joint - I just crudely shaved it off with a chisel and rounded them off with 80 grit until I could go to a lower grade of sandpaper. With all the other warts to try and conceal, this was the worst one. Thankfully, they will love this freebie and never complain but I’m wondering how to do it correctly - if there is a way. The upcoming Tea Box Won’t have any radius corners!

I like the radiuses look rather than a boxy square look. To eliminate the one angle longer than the other on the corners ,you will have to bisect the angle.
Herb

 

[Herb Stoops]

Here is a video. You have a end board meeting the side board at 45°so the inside angle is 135°. Half of that is 67.5°. This is very important to know because you are going to be running into situations all the time where you have to miter a corner that is not at 90°. This is especially true if you ever have to run trim as in a house. Most corners are not 90° and you have to make a good fit.
Also you can stand the boards together at the angle you want then draw a line between the inside angle and the outside angle with out ever knowing the actual angle is in degrees.

Herb

 

[Herb Stoops]

Maybe this will help.
When you make a normal 90-degree miter, you cut the pieces at 45 degrees (half of 90).
For this application, you are making angles of 45 degrees, so the cuts would be 22.5 degrees (half of 45).
That will make the mating pieces the same thickness.

Hmmm. Am I thinking wrong, the inside angle is 90°+ 45° = 135°. the bisected angle would be 65.5°.

Herb