Fitting my vent terminals for my Mvhr supply and extract valves and I know a few on here have used BPC for the ducting etc. When cutting the plenums down to size, the bit that sticks through the plasterboard that the ceiling valve pushes into, have people left them to protrude a little bit lower than the plasterboard? It is just when I looked at how the valve attaches to the plastic tube it needs to compress a foam gasket and I assume for this to work properly it would need the tube to be left longer than the flat surface of the p/board to make a snug fit? TIA