Avoid hourly rates whenever possible.
It takes close to $100 an hour just to keep us from losing money. 90% of the time there are two guys in a truck. That is about a dollar a minute, hard costs.
1) Justify to who? You are the one who determines your prices. You should be marking small materials up 500%. You can't sell a 50 cent switch or fitting for $1. It costs more than a dollar to write it up and account for it.
2) It's just as easy to do on big items as small ones. Use a calculator
When you get to items that cost $500 and up, you can reduce the mark up and still make good money. Remember, you have to purchase, stock and warranty the stuff you install so you spread those costs over everything you install.
On your labor jobs try to bring in $1000 per day per truck. Half of that should be gross profit.
The sooner you get past the mental block of "justifying" your prices, the sooner you will start making better money. Sit down and figure out what you will have to charge in order to reach your financial goals.
You want personal justification?
In 5 years, you want a piece of real estate for your office, warehouse and 4 more trucks. It will cost $X.00 You want a decent salary of $X.00. You want people to help you in the administrative end as well as in the field. That will also cost $X.00.
You have 1300 working days to meet these goals. Do the math and figure out how much you need to bring in daily to do so.