In article ,
charlie b wrote:
SNIP...

You see, when you go to miter the corners of two boards
which are NOT the same width, the miter angle is NOT 45
degrees on both boards. Hell, it isn't 45 degrees on either
board.

Now, were it my decision, I'd go with 4" wide boards for
the face frame, cut them at 45 degrees and be done
with it. No one's going to be the wiser so what the hell.

"But then you'd feel a lip where the bottom of the face
frame and the bottom of the top torsion box meet. And
besides, I've already ripped the top face frame parts to
3 1/2" and the sides parts to 4". I don't want a lip and
I don't want to go get more walnut boards. Let's JUST
figure out what the miter angle (sic) is for what we've
got."

THAT is the subject of the posts to a.b.p.w.

charlie b
(the guy with a son who has expensive taste and very
specific ideas for what his "piece" should look like)

The angles you're looking for are not hard to calculate. Use a little
simple trig and you're done.

For most miters, both sides have the same width and therefore the
tangent is 1 and the arc tangent is 45 degrees.
For your project where one width is 3/4" and the other is 1/4" there
are two tangents you need to calculate

3/4 divided by 1/4 = 3. The arc tangent of 3 is 71.565 degrees.
For the other angle you have
1/4 divided by 3/4 = 1/3. The arc tangent of 1/3 is 18.435 degrees.

Now I suspect that your plywood isn't really 3/4" and 1/4". It's most
likely something close like 23/32" maybe 7/32 or there abouts.

You could measure your wood and do the math. Or you could make yourself
a triangle using the same ratios as your wood and then use that triangle
to adjust the angle on your saw to cut the actual miter.

For example, assuming 3/4" and 1/4" as your thicknesses.

Cut out a square using scrap wood. Let's say about 12 inches.
Now measure 9 inches from the corner and make a mark on the edge. This
represents the 3/4" wood.
Measure 3 inches from the same corner and mark the other edge. This represents
the 1/4" wood.
Now draw a line connecting the two points and cut along that line. You will
now have a right triangle with the legs being 3 inches and 9 inches. The
two angles on this triangle are the two angles you want to cut your wood at
in order to get the miter you want.