Sorry, for some reason I'm just now seeing replies to this.
@sbmohan, as I understand it, ProMix BX and HP both have a little fertilizer charge, just enough to get things going. There aren't any hard and fast rules I follow for fertilizing and I've changed up the way I do things from year to year. If I had to give a rule, I'd say I start fertilizing once the plant has at least three healthy leaves and is holding them. I've often done a water soluble fertilizer but also like the time-release top dress fertilizer. Both seem to work well.
Rather than do it at each watering, I'll typically wait until the leaves aren't looking quite as green as I think they should (I know that's vague and subjective). That's probably not ideal though. So I'd value input from others here. A weak feeding at each watering might be best.
@Smyfigs: Those bags aren't my photos. I do not use bags, but rather direct-pot them into 4x4x9 treepots. Bags just add an extra step (and extra material) that I'm not interested in doing. However, if it works well for you, more power to you :-)