It's important to shake the unopened can of chickpeas well. The starches in the liquid settle in the can, so to take full advantage, you need them evenly distributed throughout the liquid. Drain the beans through a fine-mesh strainer over a bowl and reserve for another use. Whisk the liquid, and then measure.
If you're baking with aquafaba, you'll need a stabilizing ingredient to whip it properly. When we're whipping eggs, we often turn to cream of tartar. Happily, we found that cream of tartar works equally well with aquafaba.