I'll say what I do, but years of practice of flying close formation is where I got comfortable.
There is no one value of the throttle that will keep you in formation. You should be making tiny throttle adjustments while on the boom (or in formation period), the only aircraft in the formation that doesn't jockey the throttle is the jet acting as the lead, and who everyone is flying off of.
Hard to see me moving the throttle, but you can see the Fuel Flow fluctuate, should give you an idea how much I'm moving it back and forwards.