I've used flat rate for 5 years on residential remodeling work. It works well if it's set up well and you know to adjust for actual conditions. Like T&M it's always better to be a little on the high side. With flat rate you can't ask for more money on the work just because you quoted too low. Without task prices I'd forget the little details of non-productive time like restocking, recordkeeping, as well as profit goals.
