Is your goal to get them as close to 80" from the floor as possible, without going over? It just depends on the plant. Since they're clones the original grower/supplier may be able to tell you how much they stretch. Otherwise, since you have two of each, you could start flowering one soon and wait for the other to grow more.
I have one that grows to 70" no matter when I switch to flower, at 12" or 30". It just doesn't want to be any shorter. I have another one that generally only grows another couple of inches after the switch.
80" is a lot of height. You can grow trees in there. You should be OK no matter what you decide. Sorry I couldn't be more helpful.