6" is pretty short. I suggest you aim for 10" - 12" of stem between the roots and the screen. First, it's just difficult to work the stem to that height. You would definitely need to start LST from the early stages. Second, you need space to work under the screen for watering and pruning. It's doable but much easier if you give yourself a bit more room. The rule of thumb is to veg the plant until it fills between 1/3 and 1/2 of your screen. Then flip to flower. Once you are familiar with how much your strain stretches during flower you can dial this in. Better to under fill the screen a little than over grow a lot!