The once ounce mark is just a basic, beginners' starting mark. It's pre-primer goal, like a high diver is told, "Don't hit your head on the board— on the way up OR the way down."
Stop the pour just before it goes thin or blonde and TASTE it! That's all that counts. If you get 0.5 of an ounce and it tastes good to you, that's the real goal, and all that matters. To increase the volume before it thins, you can try a finer grind, or lower the brew pressure a bit (adjust the pressure relief valve).
On the other hand, IMO, singles are a waste. Just when you hit a good pour, all you have is about one ounce to enjoy... one sip and it's gone.. oh, so sad.
Singles also tend to exacerbate problems with grind, dose, distribution, and other such parameters. Get good at pulling doubles, get the machine tuned in, and then transfer what you learn to making singles.