How much tip during Safari: quite different opinions

Hello, we will soon go for a 7 day safari in Northern Tanzania with a tour operator. We are a group of 6 people sharing a jeep. How much per day should we tip the driver? I read opinions between 20 $ for the whole group and others (like our tour operator) talking o 10 by person, thus 60$ for the whole group. What's reasonable? Thanks in advance Erwin