Hey bro, no diff in between 20, 36, or 60 as far as I know, just plant sites.. I do use every site, experimented around with every other,etc,etc... blah.. throw up 2 x 1000w horts over that 60 plant site and blast them girls proper..
When I grow in the system I only grow lollipop style.. Just a huge solid running cola, no side branches at all. Depending on strain, I start flowering around 6-10"..
So on the solution and depth... Starting out, I place my cuttings directly into the net pots then hydroton on top of that. The roots will hang some or not, don't matter.. The solution level starting out should touch the very bottom of the net pots the cutting is in. Due to capillary action the water will be sucked up to the roots if they are not already in the solution..
After a few days+ the roots will have grown fast and large and you will need to lower the water level down, to almost the lowest or the lowest.. All the excess spray and humidity in the tubes makes the roots fine and healthy. They will eventually touch the bottom and will grow all into one mass. That's why I like to slightly raise the ends up to make it a more NFT system and help with root rot, etc. I also like to make spray holes at every site. I just get a sharp drill bit, mark and have fun..
Keep the level in the tubes at the lowest til your done flowering. I use a continuous reading meter in my res, too. I always know what going on with temp, ph, ppm, etc.. no other way imvho..
Tips: I would recommend cutting panda plastic at two+ layers deep, place that over the entire unit, cut 60 holes in the plastic while sitting over the tubes. Then place your net pots in the holes thru the plastic.. Helps keep algae out, internal heat down, easy clean up if ya foliar, etc.
I would really recommend investing in a water chiller for this unit, unless you live in a climate that's adequate for keeping the water temps chilled. Root rot will hit ya and kill your whole crop in a matter of days/hours if your water temps are too high.. This is the most important issue with these units.. water temps!!!...
This is a serious unit, it will give you serious results unseen by most. However, you have to learn how to harness this beast! It has a very very very low buffer tolerance. In other words, one slight fuk up and you could ruin you whole crop. It's not like soil-soilless- or even coco.. Those have room for error, this is a lot less tolerable.
Happy harvests and have fun, it's a fukin' blasts once you get the system dialed in.. There's nothing that can run with this system, nothing.. HANDS DOWN THE FASTEST GROWING SYSTEM THERE IS OUT THERE, IMVHO!!