How's it all going Archie ?
Trouble is with the growing books, most (all ?) of them aren't about autoflowers or LED lighting.
As my flightless-bird friend said, epenguin and i both use the 120W Prakasa and it does a very good job for such low power, easily beating my old 250W CFL, so your light is not the problem.
The food of the plant is the soil and the nutes, you HAVE to use quality soil. I use Plant Magic Soil Supreme from here :
http://www.plant-magic.co.uk/products/supreme.html
but any light, organic compost that is a good brand will be beneficial.
I also use the organic nutes from the same company.
Organic compost and organic nutes means no worrying about Ph and all that crap.
I like things to be easy and growing organically takes a lot of chemical guesswork out of the process. You can use your water straight from the tap, my water has a Ph of over 8 and it stinks of chlorine/ammonia but i don't have to worry about it.
So, to sum up :
1. Get a good quality organic soil that is light (enough nutes to last only two weeks).
2. Get good quality organic nutes.
3. Follow the nute manufacturers guidelines but use half-measures and ask advice here.
Can't believe you've done a dozen grows and you've not been happy with a single one yet
My autos always hit about 45cm (18 inches) but my pots are only 3L.
Also, because ALL autos have the Ruderalis gene :
https://en.wikipedia.org/wiki/Cannabis_ruderalis
they will be shorter than photos. But growing in 11L buckets you should have 36 inches or more.
Get some good soil and nutes mate!
All the best,
steely.