Another tip on deciding when to transplant --
Look straight down on the pot and note where the widest leaves are in relation to the side of the pot. As long as the pot is wider than the leaf stretch your plant is using most of it's energy to grow roots to support the plant. Roots usually reach the inside wall of the pot when the leaves are as wide as the pot. So, once the leaves are wider the plant will re-direct it's energy into veg growth and the plant will start to fill out. In solo cups I usually wait until the leaves are 150% the pot width before transplanting. I go from the solo cup to a 1/2 gallon pot so the roots can quickly fill the new pot and start vegging again. I end up in 3-gallon pots as I grow quick, smaller indicas.
Good luck, BigSteve.