The height of the legs as a whole is up to your preference based on your height and comfort. But the incline of the game should be 6.5 degrees. If you don't have an inclinometer, you can download an app on your phone (I use "Clinometer"), position your phone on the playfield (not the glass) a few inches higher than the flippers. Try to use the long edge of your phone that has no buttons.

It should read 83.5 degrees on the app. If it's a higher number, adjust the back legs higher. If the number is too low, either adjust the rear legs lower or adjust the front legs higher, your preference. Also, be sure to use a level on the playfield to ensure it is even from left to right as well.