Thanks Dustin. Good to know 50mm works OK. I was thinking of an upper vent triangle on the high end wall - but figure I could cut this in later if necessary.

Height: - yes, 950 is low. It was an arbitrary number. I am thinking more toward 1150 or 1200mm. I like to be able to sit fully upright. This is pretty high so I don't want to have to jack it up any higher to compensate for sag or cat curves.