wooden three-tiered strawberry bed?

Question

What is the formula for determing the length of each side of an octagonal shaped strawberry bed if opposing sides are five feet apart? Since it will be a three-tiered bed, made of 2 x 10, how long will the sides of the second level be, for it to be one foot smaller than the bottom level, and also the third level?

Answer 1

I have no clue, but a tedious way would be to square the intial boards to each other and add 6 more, that converge at the same angle. Then mark and cut, fit and install. Repeat with successive layers.