Expected tip for driver of rented car in Sri Lanka

I traveled around Sri Lanka in a rented car with driver.
The contract has this note:

Above Price Not includes
Driver Tip – We encourage our customers , whenever they wish, to offer drivers tips for satisfactory services. Tipping drivers is accepted practice, and it is especially beneficial on long tours when drivers may be away from home for long periods

  • About the driver, the contract only explicitly cites “Driver Meals and lodging” as included.
  • I am not particularly demanding, but I am perfectly happy with the service.
  • Coming from a no-tips country, I have no remote idea what the driver is used to receiving.

Is there a rule of thumb for this kind of tip?
For instance a percentage of the sum paid to the driver’s employer? (USD 250 for 5 days)