Do you know the formula or how to calculate?
No.
https://bitcointalk.org/index.php?topic=320959.msg33423133#msg33423133I would like to know how much to spend to decide, I don't want to just start spending and hoping for the best before knowing exactly how much.
What is the point if it is going to cost me half of what I got?
That's not how gambling works. Try thinking about how much you might win.