Hmmm i would say if you use like a mountain dew bottle, not those funny shaped coke bottles, i personally would put the by the bottom of the label or maybe a few centimeters higher to account for the weird shape bottom. Haven't run a hempy 2-liter yet cause i was using the 1.75liter juice bottles i had on hand and they are square so its a little different but I put the hole on those about 2-2.5 inches up. Im sure you could get some really nice colas in a 2-liter with proper training. I am fond of lst while growing in 2-liter but that was more for the fact i use cfls then anything to do with the 2-liters lol.
plant on the left was grown in a drain to waste 2-liter in a mixpeat moss soil less mix and lots of added perlite. I ended up having to clone that plant due issues i believe I was having with the peat moss mix throwing the ph off but its decent size for being in a 2-liter i just wouldnt advise going much bigger.