I've never worked at a spa or with an agency, so my knowledge is limited. If you are just trying to get someone to handle all the "paperwork" so to speak, so you can just focus on appointments, I would highly suggest a responsible, experienced assistant. You agree on a weekly/monthly rate. You can do a percentage, but I dislike that setup since their motivation is now to get you as many appointments as possible, and they MIGHT not be so concerned about your safety as their bottom line. I always go with a flat fee. A good assistant will help you make plenty more money than she costs, and relieve you of hours and hours of advertising, screening, scheduling, etc. You set the terms of screening standards, you set your own hours, you set your own price.. you're the boss. They work for you, as opposed to you working for an agency. Now you just keep an eye on things, make sure things are being done right, and do your appointments. Much less headache. A good assistant is an amazing asset.
Anyway, hope that helps. Best of luck!