Thanks for the replies. I was able to get this one to turn out nearly square so it worked out but if I have to do another one that fits in a rectangle space I will have to do a lot more thinking on it.
it is simple geometry. the only glitch is you need to be able to find the angle from the tangent value computed.
for example, the lower left bottom corner... the angle of the line to the upper right top corner....
tangent of angle = length of opposite side divided by length of adjacent side
i.e. tangent = 16/12 = 1.33333. there are a few billions internet sites that will tell you that the arctangent (aka tan-1) of 1.333 is 53.13010235 degrees.
plug "tan-1(1.3333) in degrees" into Bing, for example. you will find difference at 3-4-5th decimal place depending on how the rounding is done.
you can calculate the angle of the top in a similar fashion,
knowing that the three angles inside a triangle always add up to 180 degrees, , , ,
180 minus 53.13 minus 90 (the bottom right...) = 36.86989765 etc etc.
which is not approximate, or sort of, or close to .... that is exact.
the Greeks knew how to do this stuff. they refined geometry theories that go back some to 3000 yrs BC.
it's not fusion rocket propulsion as some would have you believe. the "equations" exists are are quite well known to people who are not blowhard idiots. the equations do not change every time somebody builds something.
if you have a digital protractor you can get the saw set real cotton picking close - but I'd advise cutting some scrap ends to be sure the tilt is mechanically set "right on"
the geometry and the math is really the easy part. if you have only one table saw, you can cut the length needed (ye' olde a^2+b^2=c^2 thing) - but repeating the angle cuts to "exactly the center line" is the real problem.