Labor is the part most shops think they understand, and it is the part most often wrong. The obvious mistake is to use the hourly wage of the tech and call that the labor cost. A proper labor component includes the time the job actually takes, the time spent getting to the site and back, and the cost of paying the tech for time that is not billed.
Start with job time. Pull real records from past invoices and dispatch logs for each job type. Look at the typical time, not the best time and not the worst. If you do not have usable records, have your strongest techs walk through the job and time it, then add time for the slower techs who will also take it. Do not build the book on the fastest time you have ever seen. A flat-rate book priced on the best day of your best tech will lose money on the ordinary days.
Next, account for the time between jobs. Drive time, parts runs, loading and unloading the truck, paperwork, and callbacks all consume paid hours without producing billable work. If a tech bills four hours of work in an eight-hour shift, the labor cost of each billed hour is effectively double what the wage suggests. You need to decide how much unbilled time to build into each billed hour, write that decision down, and apply it the same way across the book. For how paid time becomes pay, see Calculate technician overtime with 8 payroll fields.
Then decide how labor enters the price. Some shops price labor per hour of tech time. Others build a labor line for each job from a target hourly figure. Whichever method you choose, write the rate down, date it, and use the same rate across the book. When wages change, the book should change with them.
Watch the skill mix as well. A job done by a senior tech and the same job done by an apprentice have different labor costs and often different results. If the book sends every call to whoever is free, the labor cost on paper will not match the labor cost on the truck. You have two honest options. Price the job for the skill level it requires, or make the skill requirement part of the job definition so dispatch assigns it correctly.