LP regulator at altitude

I got certified in Denver in 78 so all this altitude stuff was covered but it's making my brain hurt trying to remember how to splain it. The reg delivers a set psig, not a constant psia, so because of this, the gas automatically derates itself with altitude. Nat'l gas in Denver is 829 btu/cf whereas NG at SL is ~1000 btu/cf. The same thing happens to LP. I don't remember if that's Boyles law or Charles law. This natural deration still isn't enough to satisfy the deration required by the AGA. An appliance built at SL is good to 2000' before additional deration is required but above 2000', the orifice must be derated for the full altitude, which is 4%/1000'. Any appliances bought in Denver are mfgr derated for Denver. The natural deration might be sufficient in most cases but I've seen furnaces in Denver that were brought from SL and not derated properly... terrible 'spilling' from the draft diverter. The mfgrs of RV gas appliances must have them derated enough to go to hi altitude w/o problems. There are some exceptions... it also make a big difference if the appliance has an exchanger or not. No exchanger isn't as critical as an appliance with an exchanger. There are a lot of factors involved! BTW, 1 psi = 2.307' of water column = 27.68" wc so 11.5" wc is NOT .5 psi. Craig