Look at what you charge and how much a guy can reasonably make by percentage when he's doing everything by your book. Make your percentage slightly less than this, as the tech will quickly outpace what you think he can do. Make sure the percentage is an upgrade from his hourly pay and continue to pay hourly but give him the option to take the percentage if it's higher. He doesn't get penalized if you under bid a job, he gets his hourly rate. Everyday he can look at the difference between hourly rate and what he takes home on a percentage and you'll have a happy tech.
Some owners that would be fine paying 25% will pay 23%, while "donating" another 2% to a bonus account. If at the end of the month there are no excessive damages or reservice work, the tech gets the 2% as a bonus. You're not docking pay for damages because it was never the tech's pay in the first place.
PS: Ken's systems works for two different skill levels on the same job. If equal skills the percentage gets split between all techs on the job. An hourly rate helper would just have the hourly rate subtracted from the commission paid to the lead tech.