If a solo practitioner, no employees and let's say no valuable office location, wouldn't your profit as an investor come mostly when you sell your practice (client list)? What assets does the solo tax practitioner have that generate investment income on an annual basis?
Regarding employees, I believe H&R Block, for the highest-billed tier of returns, pays office preparers roughly a 30% commission. But remember, employees also have payroll taxes paid on their behalf, some employee expenses (such as license renewal), and unemployment benefits when they are laid off after April 15, so your employee portion of expense should take that into account as well (meaning, you need to count more of your profit as "employee" compensation to yourself to include these extras).