The one thing to keep in mind is having a clear idea of what your KPIs or success metrics should look like. Once you have a clear understanding of that, that should be your numerator for your Goal. We give you a number of options for what your numerator can be (or success metric) for your goal. It can be code events (from any of the popular analytics platforms), buttons, or views appeared.
As for your denominator, leaving it as App Sessions should work. However, if you want to be more precise when it comes to measuring your goals, we give you a number of other options which include code events, views appeared, and more.