Ar you using the HP or not... it seems that you said both. Anyhow, I use
Promix HP or BX and I only feed about once every 12 days, but that is with a regular plant, not auto. Once you start getting the alternate growth pattern, I would start at the .25 or .5 concentration that you mentioned. Since I only feed on occasion, you might want to increase the frequency of feeding but with slightly lower amounts until the plant gets larger. I like to run at least water only watering in between feedings, although I usually water daily once they get pretty big (around 6 ft or so). Don't overfeed it andyou should be fine.
Promix makes it almost impossible to overwater, so your main worries should be nutrient related.