A back-of-the-napkin formula is:
(per cpt reimbursement) x (volume) x (your negotiating strength coefficient) = offer.
However you are doing it wrong.
First, ask not what the 'ballpark range is on what people are offering'. Instead focus on what the TERMINAL POTENTIAL is.
Second, get rid of the 'salary' mindset. You are the one who should eventually be owning the shop and paying yourself. Your goal should be the coefficient of 1.