I just lay it out with mat board strips that are the width of the frame, till the balance is what I want, then measure for the lengths that will be needed.

Don't have Paul's book in front of me, but a easy formula:

360 ÷ by # of frame sides ÷ by 2= degree of cut.

Example:

Four sided, 360÷4=90÷2=45

Eight sided, 360÷8=45÷2=22.5

Assuming that you mean the diamond from playing cards, you have a 4 sided shape with equal length sides, and 2 different angles.

All of the angles in a 4 sided shape should add up to 360 degrees.

As you have 2 of each angle, adding 2 non equal angles will be 180 degrees. Eg 80 and 100, or 60 and 120.

These angles then need to be divides by two to get either side of the mitre, and then subracted from 90 degrees to get the angle needed for the cut.

For a 60 degree angle, you would need to divide by two, then subtract that from 90 degrees. 90 - (60/2) or 60 degrees on a mitre saw.

For a 120 degree angle, you would need to divide by two, then subtract that from 90 degrees. 90 - (120/2) or 30 degrees on a mitre saw.

Most mitre saws cannot cut an angle of more than 50 degrees without doing some tricky things.

I've made it work on a 30, 60, 90 triangle frame by adding a 90 degree block, and then offsetting from this.

Not the easiest or safest way to cut a frame.