So you are talking about root mass on new clones?
Clone roots can be wishy-washy. I only deal with a few clones at a time, and I use the water cup method. Once it shows roots, I put it in soil and and build root mass by transplanting at the right times and proper watering routines. Someone else may have some pertinent info relating to your method, but I have found if if you just give the roots a good environment, they will thrive.
Also, are you saying a 7" pot is your final size, or gallon?