If you figure an OG of 1.050 for a 5 gallon APA, then you should ideally make a 1.5 liter starter with around 150 grams of DME. This should get you around 175 Billion yeast cells needed to adequately kick off the fermentation process. You can pitch what you have made and still be okay.
If you want to hold off a day, then you can just add some more boiled DME (cooled of course) to what you have already made, to get closer to 1.5 liters in total, and brew tomorrow. You still achieve your goal of making beer either way, and people brew all the time without starters. IMO, it is good to use one, and required with higher gravity beer.