From what I've heard about Promix BX, it contains lime fresh out of the bag, and is supposedly very accommodating in its ability to maintain a desirable pH. So, for at least the first few months, it should be buffered well enough not to need the addition of extra lime. Promix is a soil-less medium, containing only peat, perlite, and vermiculite, so what I've gathered is that all soil-less mixes need to stay at a pH of around 6.0... 5.8-6.4, if I remember what correctly what StinkyAttic said about it.

Take care.