The full service contracts is a bit special issue.
We have some number of them and usually in the contract, the range is strictly defined.
In your case, till the end of the factory warranty, the situation is pretty safe. You make the inspections, dealer makes the repairs.
If dealer claims the misuse or overwear, you can refer to this.
The other situation is when the units are after warranty.
There will be always some risk, but it's worthwhile to take it.
Simply specify the terms in the contract. What is excluded from monthly rate (like steering axle, when the terrain is rough, etc.)
We have elaborated the monthly rates for different trucks, applications, periods etc so now it's a routine.
The rates are based upon the factory data and experience, but the basics is that the total monthly rate (inspections and repairs excluding tyres, batteries, big attachments), for standard, not very "cruel" application (like in-out cold-store) shouldn't exceed x 2,5 rate for normal factory defined inspections.

It means, that if the monthly rate for inspections is 100$, the full service rate shouldn't exceed 250$.