if you have any of the rock it will spread, make sure calcium and other levels are up to par that's whats going to help as well as the places with high flow atleast in my tank have the most
purple up is a waste of money .. don't buy it .. just keep yur calcium between 450-500 .. and alkalinity to a level to even out the calcium .. can be like 9-10 dkh if im not mistaken .. also actinic supplementation does the job too .. its funny though when i made my first reef tank .. i did the same thing .. ask for scrapings .. and got them and still never had any coraline spread .. when i became slightly more advanced with my second setup .. i didnt bother wit that .. and coraline grew on its own .. and pretty fast too with the right conditions in the tank .
so don't worry too much about it .. just put in some Live rock that has a little on it already and it will spread on its own .
Yes ,I have heard the same thing about purple up just as long as your level are right there is no need for it....I think I will take your advice and let nature take its course ...Thanks Sal