Seat - Back rest installation measurement

[abl1111]

Kind of a silly question, but is there a definitive measurement for mounting a back rest to a horizontal bench ?

I'm mounting a removable backrest onto my i/o engine cover and am not sure how far I should mount it to accommodate for all sizes and shapes ( adults, kids, tall, short ) - taking into account how legs should feel at the bend in the knee etc...

Anyone have a knack for this thing - or is there a standard ? I have all the room I need, just don't want to make it too shallow or too deep.

Thanks