You can't get a percentage of parts in pay. This would encourage techs to change unneeded parts to get more money. I am about the same in service area and area rates change as milage increases but we do not charge for gas,that is part of the Service call. I include 15 minutes time with the call and then 60/ hr or 15 for every 15 Min's. I am the lowest in my area, but have had customers and many referrals for 30 years so it all balances out. Most of my calls are within 10 miles, my service charge is $65.00. It goes up with milage out up to $90.00 Even higher up to $150.00 for my specialty of Antique gas stove repairs, and 80-100 per hour after the first 15.There are only 2 of us north of Golden Gate that do these. A very good specialty.