Driving – Expected tip for drivers of a rental car in Sri Lanka

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

About price Not included
Driver tip – We recommend our customers to offer driver tips for a satisfactory service whenever they want. Tipping for drivers is a recognized practice and is particularly beneficial on long tours when the drivers are away from home for long periods

  • The contract only expressly quotes about the driver "Driver meals and accommodation"as included.
  • I am not particularly demanding, but I am completely satisfied with the service.
  • I come from a country without a tip and have no idea what the driver is used to.

Is there a rule of thumb for this type of tip?
For example, a percentage of the amount paid to the driver's employer? (USD 250 for 5 days)