It's the volume of the pot not the depth of the soil that matters. 1 gallon per foot that the plant will finish at.

Cutting the bottom off a pot is not how to transplant. You need to tip the pot on its side, rolling it and gently tapping to loosen the root ball, then slide it out into a pot that has enough soil already in it to make the top of the old soil even with the top of the new pot.

If you have leaves browning you need to figure out the CAUSE. Could be lots of things. You can cut them when damage is over half the leaf, or let them fall naturally, your choice. Just make sure to get dead leaves out of your grow room otherwise you get disease.