Depends on the snake and what gene he takes after for lack of a better term. Caesar is 37.5% SD, 31.5% dwarf and he is just over 7' at 18 months old. He was also fed pretty heavily as he would push hard as a baby if not kept full. I personally think the best judge of the size is to look at the parents. I got Caesar from Kris over at Vital Exotics and his parents were 6' dad and 8-9' mom and like i said, he is 7' ish. Kris figured he would top out at 7' with 'normal' feedings and even with my heavy feedings, he is still around 7'.
I personally would probably shoot for something in the 50% SD range. 75% SD in some of the rarer morphs might be really hard to find as well as costing you an arm and a leg and your first born child. Cant hurt to look though. Like i said, i would join the Super Dwarf retic FB and ask there. More people there with retics to pull knowledge from, plus Jacob aka JMCrook is a member there as well as me. I thought Steve was a member there too but i checked and he isnt so i put in a pending invite for him lol. The FB page is called Super Dwarf and Dwarf Retics-Keepers and Breeders.