dont over think this it's nothing of an exact science. just lay a board on the bed of your jointer or planer and adjust the roller slightly less than than what the board coming out will be like splint mentioned. ive tipped many stands over being set too high. your gonna have some boards come out curving down and some may slightly curve up. the vid he shows a continuation of the bed, not so with roller stands that might be 2 or 3 feet away from the machine.

working with my hands is a joy,it gives me a sense of fulfillment,somthing so many seek and so few find.-SAM MALOOF.