One tick/tooth of the grooved/splined shaft on the servo usually is close. The more you lean it over at center, the more right/less left you get. Obviously you can over do it.

The reason this needs to be done is the servo end is higher than the bell crank end of the link, it goes down in each direction, forcing the wheels to the left a little each way. By starting down a little it balances out better, takes away some of the left, and adds to the right.

You can also move the servo slightly left to right on the 4 mounting bolts on the little posts to help center everything, in addition to adjusting the length of the link (do first) going to the servo arm to get back to center.