Yes, that is all very interesting except the OP asked for guidance on whether a 15l cylinder is a worthy contender for his next acquisition. It is. Perhaps you can explain how a rubbish mix like 80% can be considered a 'standard gas'! Scapa is nice easy diving which is rarely blown out. You'll enjoy it, although two stages seems a lot of work and quite costly on gas! For maximum crew appreciation you should side-mount all your bottles For what it is worth gas costs for a week of CCR diving in Scapa, doing to 2 x 90 minute dives per day, is likely to be around £30 (assuming no bailout is required).