If you add the optional egg in Step 3, do you reduce the milk to keep the liquid volume to 3/4 cup?

Good question. It's not clear from the recipe, but I would recommend reducing the milk, otherwise it might end up too wet. Since the egg is a bit denser than the milk you may end up having to add a few more splashes when bringing the dough together, but better to start with too little liquid than too much.

Biscuit dough is generally not so fussy that you have to be exact on liquid amounts. One egg isn't going to bring that much liquid to the party that it will upset things. If your dough is a bit sticky, it will just pick up a bit more flour when you turn it out and shape it before cutting. Flour can also vary on the amount of liquid it will absorb, depending on how humid or dry the environment in your kitchen is/was. Don't worry too much about it. Biscuit dough that is sticky bakes into moist, flaky biscuits. Dough that is crumbly or too dry gets you a dry puck.