The problem with that is, there are different levels of trouble shooting, that is why you can.t include the dispatch fee with the trouble shoot, we tried that years ago and always lost out, some trouble shoots like an outlet not working, can be an easy single room one outlet or the mullti room multi outlets, a big difference in the time requires to trouble shoot.
I use a similar system.
The Service Charge is to get the truck w/tech in the driveway.
The Troubleshoot Charge varies with the problem. There's a different rate for a single device, or a circuit, or a multiple circuit. I just simplify it into Level 1, 2, 3, etc.
Then there's the Repair Charge after you know the problem.
BTW if I'm troubleshooting a circuit and find the problem in the first device I usually discount to level 1. If a multiple circuit problem is the utility I discount it to a level 1 or 2. The idea is if you spend 3 hours digging in attic insulation to find multiple buried HO splices, you're covered & not going on a $50-100 service charge with troubleshoot included.
Also, if you didn't charge enough on the troubleshoot charge you can always add it to the repair charge. Working at a loss isn't a good option.