I second/third the comments about the importance of the seat position as well as @Muskabeatz comment’s that it’s very doable by yourself (though admittedly a PITA of sorts).
That said, if you’re not being scaled with the kart in order to find your seat position (e.g., the ‘full proper way’) then the only real choice is to set it to whatever the manufacture recommends for someone of your height and according to whatever the category of the kart is (in your case shifter). By going with the manufacturer recommended setting you can at least be sure to be in the general neighborhood. This thread imo is possibly the best place to find seat installation info.