Decks, Patios, Porches and Docks - How long of deck board for diagonal decking?

I wanted to run my 5/4 deck boards on my 16" OC joists diagonally as close to 45 degrees as possible. My deck is roughly a rectangle with cut corners and an angle that goes out from my 10' deep section to my 12' section. My question is, how long do my deck boards need to be to span the 10' and the 12' deep sections without having to create a butt joint by adding another decking board onto the end to make it span the entire distance?

Of course, I am wanting to keep the price as low as possible and I'm wanting as little waste as possible. I'm thinking that the 14' are the most cost efficient at my lumber company. Would I be able to get a 14' to span the 12' section on an angle close enough to a 45 degree?

What are the best lengths to use for this deck? Would there be alot of additional cost versus running my boards parellel with the house?

take a measuring tape and run it at the angle you want across your deck. i'm not trying to make light of your question, but it's a heck of a lot easier to measure it than to try and do geometry. another idea is to get a piece of graph paper and use a ruller with 1/4" equaling 1 ft.

DO NOT put 5/4 decking diagonally on joists that are 16" O.C.!! You are overspanning the decking!

5/4 decking can span 16" MAXIMUM. Do what jay_myself said, and lay your tape diagonally from corner to corner of the deck. Now measure the span between the joists along the edge of your tape. THAT is how far your deck boards are spanning.

Solution is easy -- add another joist between each of the existing ones.