Yes, I've used the search, because the question has been asked so many times it takes a bit of time to read all the threads. Sometimes you have to work at things.
Quite a lot...and that involves what youre thinking of too....
To answer the question, you need to run whatever backspacing / ET you can get hold of to keep the scrub radius / (often incorrectly termed the offset) the same, as otherwise its gonna be detrimental to handling.
Its impossible to give a definite answer as wheel backspacing's vary and that means the overall backspacing / ET you have to run is dependant on the design of the wheels!