With mileage, you know up front what you are going to make, ie, if your contract reads .85 cpm, then you can eaisly figure out what you made after your trip with expenses.
On the percentage side, your contract will specify what percentage of the tariff (what the company booking the freight is charging the customer) you will receive, i.e, 60%. 70% etc. If the company is charging the customer $1.40 and your contract is 60% of the gross, you would make $300.00 on a 500 mile run.
Now, how do you know what the trariff was, mostly you take the word of the company. As I recall Federal law says that you have a right to view the billing statements. Hope this helps