Hi Nick, if you’re overlapping long ways (i.e. running the coloursteel into the polycarbonate or vice versa) then you’d need a 300mm overlap. If it’s beside it, then the standard 1.5 corrugation overlap is fine.
In most cases, putting the polycarbonate on top of the coloursteel or corrugated iron is better. This is because the polycarbonate expands more than the steel so this will allow it freedom of movement. It will still need the oversized hole for the screws.
Obviously if the coloursteel is higher than the polycarbonate, and is running onto it, then the polycarbonate will need to be underneath.