w51, there're many ways to skin the cat but I find it simpler to have the same denominator, if you wish, when working in other systems let it be metric or QFE. Historically QFE is used in the metric system, except UK RAF, thus since you're already using QFE (height) why not simply set given height in meters. You don't need brainstorming, converting etc, just set e.g. 500m on PDF, can't be that difficult, can it? Yes FCU is only ft but you're supposed to cross check the settings with PDF anyway. The metric system and QFE are two different animals but used to form the same system in the former USSR however QFE and FT were very much used in the UK. Whether you set meters or feet doesn't change the fact that one might be at the wrong height, altitude if the wrong pressure was set. The confusion comes from the fact that multiple steps are necessary to convert given metric height QFE based into FT altitude QNH based coz then you must first convert QFE given metric height into QFE ft height then add field elevation to obtain QNH alt but it's all in vain if you set QFE instead of QNH on FCU and vice versa. Why not make it simple, if QFE is used select height, if QNH is used use altitude. Doing so you'll always have the same denominator whether you set meters or feet or inches etc.
