If your competing then yes 15 feet is the distance to set your project at.

In terms of the P-47 that would be a full scale distance of either 75ft for 1/5 scale or 60ft for 1/4 scale. What you can see at those distances from the real plane is all that you should worry about replicating. The diameter of rivets the weight (or call it thickness) of the panel lines, the amount of wrinkle in the skin surface, anything at all if you can see it and replicate it that is your goal. If you weather a plane more than that and a judge scores it high then its not being judged on scale accuracy merits its more emotional than technical. So if your competing then you have to play that game.

For a personal Sunday flier and to practice technique its good to learn to approach the project from a solid reference point and follow through with it to learn how to replicate that scale look. It will be a head turner regardless of intent to compete or not, when its done right it just looks right.