How have you leveled in the past, with slide out or in??? Use past experience to set up the Level Mate. If you did it before with the slide in, level like before, and when slide is out, recheck the device for difference. You'll get a good feeling for the best setting after a bit of use. Perfect level is almost impossible to achieve if you're a purist!!!
FYI, I level with slide in, with a bit of an offset to accommodate the slide when extended. Also still use bubble levels to verify. And they're not perfect either!!!