I doubt that you know that you have a 40% chance of winning the stake because if you did, you should of been able to calculate the x.
Also if you want to 50/50 odds, just stake it on a max main?
To answer your question though, there are many factors that come into play when calculating odds (Accuracy/Hitpoints/Max hits/Strength level). There are applications out there intended to calculate odds but truth be told, that information is valuable as staking is very competitive and you would be fortunate if you had someone to help you.