I wouldnt plan on any baby retic no matter then genes to only get 6-8'. Even pure Kalatoas can get over 8'. Look at Phyllis, JMcrooks pure female Kalatoa. She is almost 9'. And my 37.5% SD 31.5% dwarf 31% mainland male white tiger is 7'ish. Personally, i would plan for a 10' snake if you get a high percent SD. That way you dont have any unpleasant surprises as trying to rehome a grown retic is going to be much harder than something like an anole
Look at the other thread to see some answers but short answer, look at the parents. That will give you a good idea of the average your snake 'should' get. But it isnt a guarantee. Plus genes like tiger tend to make bigger snakes.
My advice is focus more on the snakes personality. Caesar is a total lounge snake that is very chill. So even though he is 7'ish, he is still very easily moved and seems to like attention. Now the same size snake who is defensive and strikes, can make that 7' feel like 20' lol. Caesar is only 18 months old so we will see how he is come breeding season. Hopefully, he doesnt turn into a hormonal monster.