How does flat rate pricing work for a plumbing company?

The way I go about flat rate is mainly how it is presented to the customer. When people say flat rate it means an up front, complete price. Our goal is to keep actual costs under that number (profit). 

calculating that number to charge the customer is easy said-- hard to do accurately, unless you have a really good idea of your costs, labor costs + material costs + margin for profit and ovrehead = customer price