Sealth scrog how much vertical space required

Hi folks
First time grower here and I'm trying to set up a stealth grow using 2 stacked rubbermaid bins and the scrog method with CFL's. After doing some measuring, I'm concerned I don't have enough vertical space. My space only measures 21 inches from the top of my soil to the lid on the upper bin. The light fixture and bulbs take up seven inches of that 21 inches, so there is only 14 inches of space left below the bulbs. I've seen where someone with a hydro setup had his screen only 4 inches above his plant so I know the screen can be set pretty low. My question is, how much space would typically be needed above the screen for a scrog grow. I'm guessing that distance is going to vary depending on factors like what strain I grow etc, but I'm looking for an idea of the avg amount of vertical growth once your plants start to flower.

It would be tight, but I see a bunch of micro growers getting decent harvests from their small spaces.
Scrog, and LST, also a little super cropping where needed.
Give em lots of light !

Well I just transplanted my first clone into my grow box the other day and here's what I have so far. I haven't finished with my wiring but I'll work on concealing my wiring when I'm done.

I started off with one exhaust fan with a DIY carbon filter but soon discovered heat was going to be an issue so I quickly installed another fan and filter. I have a 3.5" intake hole in the back side of the bottom bin.

The next problem was low humidity. I already had a small bowl of water inside so I took and old dish towel and hung it from one side with one corner hanging in the water bowl. This seems to have solved my humidity issues, assuming my gauge is accurate. I know the thermometer is off by 2° high but I don't have a way to verify the accuracy of the humidity gauge.

I've been trying to get a clone of my other outdoor plant started but am not having very good luck so far. The first clone (shown here) turned out great and is growing well. The other two clones (from an autoflower strain) are not doing well at all. It had already started flowering before I took my clone cuttings. All of the old original growth has died off and the new growth is nothing more than a bunch of very tiny leaves. They just aren't developing any roots. I don't have a separate cloning box/area so I had to move them into my grow box once I got it built.

The way it stands now, I need to get a larger 12v power supply and install my circulation fan. I'm running my lights at 18/6, my humidity is holding between 40-60%, my temp with lights on is running 76-80° and 71-72° with lights off. I'm about to add 2 more 100w equiv CFL's and I'm working on some extensions that will allow me to lower my lights for now. The watering tube works really well and I have two 4 gal pots sitting on blocks so they are an inch off the bottom of the bin. Any excess water from watering can run out the bottom of the pot into the bottom of the bin and help keep humidity up. I never found where anyone stated where to get mylar so I bought some mylar baloons at wallyworld to line the inside of my bin with.

Any suggestions for improvements are more than welcome as this is my first time growing.