Forget the hackle guage. The tips of the proper size hackle should extend to the back or furthest point on the bend at the vrey rear of the hook. As to density, traditional English flies are much more sparsely hackled than you will find in the typical American Tie. flybugpa, comes closest to the correct hackling in the English tradition. Although soft hackle is not intended to float the fly it may be necsessary to use a denser hackle to allow the fly to drift higher in the water column when fished upstream. A more sparsely hackled fly on the same weight hook will generally fall faster and fish deeper than the more heavily hackled fly of the same size.
British flies have always been somewhat more deicate than American ties. When dubbing for example they refer to "dirtying" the thread with the dubbing. It is not at all unusual to see the tying thread through the dubbing as the dubbing is just that sparse. For this reason if you are contemplating tying "traditional" pflies according to specific patterns, pay careful attention to thread color as well as dubbing, hackle, and of course hook selection.
If you are tying flies for fishing, tie something you like, have fun with it, and take it down to the water for a consultation with the fish. You may just find the next secret fly!
Hope this helps,
George