I am probably going to embarrass myself now, but I have a mental block with the ISA deviation calculation in the original question. Everyone is using the PH to calculate ISA deviation at Echo. I don't see (or have forgotten) how pressure affects the lapse rate - can someone explain why the PH is used rather than the elevation? The actual number of feet from MSL to Echo is still 3480', so shouldn't the lapse rate be applied to this figure, not 3240'?
Sorry if I am being dim.