There's a basic formula where you have to add 1/2 of the initial stack proportional to the initial bounty, but it's not exact and perfeclty accurate.
There's a discussion where this number should chance depending on how it will affect our stack and the ability to knock out other players as well as ICM when reaching and/or in the money.
So basically the real number is something between 1/4 - 1/2 of the initial stack depending on the situation (it's to hard to generalize).
I've made some videos about it, you can check it out in here:
https://www.runitonce.com/users/gusma/