Parts and labour shouldn't be an overhead, you sell parts and labour at a profit, that's how you make money. We work on an 80% productivity for mechanics,more is a bonus, any less and you are not making money - we don't do flat rate in NZ, we charge by the hour, and pay by the hour. 30 years ago (1984) I earned half the charge out rate, got 10% of the workshop profits, and had full use of the company ute, and generally averaged 100% working sole charge, although some weeks were not so good. These days I'd be lucky to earn a 1/3 of the charge out rate (he has far more overheads, but is freehold), I see no perks and pay my own way to work.
Much, much harder to earn a living doing this these days.