KPK - thanks for the pic. The rock the acan is on is roughly oval in shape, and is currently "filling" the rock to the edge. So I wherever it ends up, I will make a donut shaped ring of epoxy to ensure full contact between the rock and the reef, so there is a continuous and smooth connection all round.
Clay - thanks for your input. It will certainly brighten up that end of the tank.
I'm trying to find a picture of a mature colony - so I can visualise what to expect in terms of both shape and size...... anyone care to share?