If you want the maximum pleasure from the sport, be patient, because the more difficult the trade, the longer is the apprenticeship.
On entering freshwater, 90 percent of the retinas [of Coho salmon] are dominated by porphyropsin, or red visual pigment. ... Initial preliminary microspectrophotometric testing of the Atlantic salmon has shown that their retina colour pigmentation closely follow that of the Coho.
In short, when the water level starts falling, or if the water temperature increases, reduce your size of fly. Conversely if the water height increases, or the water temperature takes a downward turn increase your fly size accordingly.
Wood discovered from his fishing that the speed of the fly in relation to its size was of paramount importance. For instance, he found that the only effective way to fish a small fly was to present it at a speed which would be natural to a creature of similar size.
In my opinion most anglers change their flies too often, generally because they lose faith in it. Moreover, when they change flies it is normally for one of a different colour, and not size, and I believe that this is a great mistake.
In fact he [A.H.E. Wood] did not think pattern made much difference, but considered size of much greater importance. J.A. Hutton asked him why he only ever used two flies, a Blue Charm and a Silver Blue. Wood replied that he didn't care which he used. J.A. Hutton then asked him why then he did not use a March Brown, a fly not commonly used at the time for salmon fishing. To this Wood replied that he would use nothing else for the rest of the season, which he did - and took the same number of fish as he would normally have expected to catch during a season.
The main advantage which tube flies have over other flies is that they can be 'queued'; this means you can create a fly of any length, colour, and weight.
Whatever practices you follow when making the 'D' loop, all movements should be continuous and smooth: these will load a rod progressively, while jerky uneven actions cause irregular loading.