Andy, If you are looking for a mix for your front wall, go 1 cement, 6 soft and admix. We find Febmix the best either liquid or powder.
Without getting technical, what Stuart says is about right. Years ago we used to use a 1-1-6 mix but the lime was only in there for flexibility, workabilility and to put a bit of fat in the muck. Regard the 1-2-9 mix, it has now been proved that this was not a good combination as cement is hydraulic, high calcium bagged lime that you buy from builders merchants is hydrated and non hydraulic, consequently, two different sets and crystallization.
oldun