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?

Answers:
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.