How can I scale down a high-yield recipe that calls for a single egg?

An alternative that wasn't presented in the link above would be to mix the white and yolk together and measure the appropriate amount of egg by volume. A large egg consists of about 2 - 2 1/4 Tbsp. white and about 1 Tbsp. yolk. The whole egg, mixed, would total about 3 1/4 Tbsp. (which would be 3 Tbsp. + a little less than 1 tsp. - or just about 10 tsp.).

So for a half egg, you would use 5 tsp. and for a quarter egg, you would use 2 1/2 tsp. If you are cutting a recipe to 1/3 or 2/3, I would recommend just going for 3 tsp. or 6 tsp. respectively.

Aside from scaling down the volume of the whole egg in your recipe, the next best idea from the link above is probably the recommendation to go by weight (if you have an accurate food scale).

Also worth noting: Whenever you scale-up or scale-down recipes where ingredients are not specified by weight (recipes that use, say, cups of flour and sugar), the recipe might not turn out quite the same as it would have if you had made a full recipe. Recipes that don't specify weights are typically not very robust. If you're going to scale-down, you'll probably have better luck with a half-recipe than you would with a quarter-recipe.