That tractor figured over it's useful life and fuel, etc, will likely cost you $45/hour of life expectancy just to own it. My point being, you need to factor in the cost of tractor ownership to your price per hour. You may need to repair or buy a new one some day when you've used this one up and you shouldn't be working to make "extra money" while taking life of your equipment away at a rate higher than you're "making" (unless you really need some quick cash).
Example - If I need to make $25/hour for my time as an operator, I'd start at the $45/hr cost of ownership, then add my $25/hour labor charge if you see where I'm going with this. Otherwise, you're treating your equipment as a sunk cost and not even covering the fact that you have to own this equipment to be able to do these services. You want to add equipment to offer more services? Then you need to charge more early to save up for the equipment.
I don't do this for a living and am just throwing my opinion out there. I'd probably charge a flat fee of $100 locally just to bring the equipment to the site (covering my loading/unloading time, gas for the truck, and time on the road), then charge $80 to $100 per hour to cover ownership and wear and tear of my tools, my time, my insurance, risk, and my put back for the next attachment or repair or new tires, etc.
These numbers are real fuzzy, but I'm just using them to make the point. You change the numbers however you see fit to match your situation. Keep in mind, autoshops charge $70-$100 per hour for a mechanic's time. Sure he has tools and investment in those tools, but your tools are a lot more expensive.