I'm assuming you have no business interest in the spinoff or they wouldn't be spinning it off. Otherwise it could be structured as a subsidiary to ring fence the business activities and potentially provide tax advantages. Assuming they want a clean break and you agree, and you want no further operational involvement in the business then a spin out would be appropriate as long as they aren't competitive or part of an infrastructure you might want for your main business.
The equity you retain should be reflective of the risk of the spin out venture. You say they have 30 customers and I'm assuming they are existing clients of your main business. If those sales (assuming they are actually paying) open the market then you have a right to claim more equity as the risk to the spinoff in forming a business is lower, and it improves their chance of raising growth equity. If the customers were grandfathered in by the existing business and so the market case is not fully proven then there is significantly more risk for the spin out and your equity is lower. You have to form a judgement. I would say high risk would justify an 8-10% equity stake and lower risk 10-15%. You're going to get diluted but that is fine because as time rolls forward your influence on them will diminish anyway.