How to manually calculate an arb?

[mybookies]

I was wondering how exactly is an arb calculated.

If I had one bookie offering odds of 1.4 for example, is there a quick calculation I can do to determine what odds it should be above at the other bookie?

 

[musk]

(1/0.4)+1=3,50 break even
if you have 1,20 you have to do: 1/(1,20-1)+1=6