How do you do this in excel?

[ImNew]

So I have an arb calculator in excel that looks something like this:


Let's say I already bet $600 on -120, instead of earning $11.1 regardless of results, I want to make a bet on +125 that would give me a more return if favourite covers, but won't lose me anything if they lose. How do you programme that into excel?

In other words, how do you figure out that you should bet $479.91 at +125 if you already bet $600 on -120?