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

Recipes for muffins and the like often involve eggs and the recipes are usually designed for a yield of about 20. I have a small oven and only want/can make about 5 muffins at a time. What do I do with recipes that have a yield of 20 or 10 muffins, but requires a single egg? Should I beat the egg in a bowl and use a proportional amount, or are there any special rules to be aware of?