Staff expenses, including your salary, should not exceed 35% of the total school gross each month. Each new hire cuts into that amount.
A good rule of thumb for a well-run studio is that it nets between 40- to 60-percent of the gross annual income. That is hard to do with paid employees. Of course, that ratio may be affected by where your studio is located.
For instance, a studio in Oklahoma is likely to net more than one in Washington DC due to the lower overhead.