If your clients have a relatively stable practice, you could add up the income you collected for them over the past 36 months and add up the fee you charged for collecting that income over the past 36 months [don't include consulting income here]. Divide the total fee income [for billing only] by the total monies collected [for the doctor, from billing only] and you will get a percentage. If your client's practice stays relatively stable for the next 36 months, you would "earn" roughly the same amount of money if you charged him that percent rather than your fee [for billing only].
I did do the above calculations.. Yes I could arrive at a % but no where near what I would charge if charging a percentage. And again the above calculation and the arrived at % MIGHT be somewhere in the ballpark (not for the example I used) but it's STILL not taking into account the AMOUNT of work done on the account. My monthly fee is constructed using an hourly rate between $25-$40 per hour (BILLING) If I billed on a % basis I would need to bill somewhere in the ballpark of 18% to be compensated for my time which then makes the % based model MOOT!