I am very new to GD&T and currently working one of my first drawings for two parts (see simplified drawing below). For accurate alignment during assembly each of the parts has two locating features (A: two slots, B: the counterparts to A's slots). After assembly, the distance between fits F1 and F2 and their diameters are the basically the only dimensions which need to be controlled to (somewhat) tight tolerances, two gears will be fit at those features.
Since the parts are quite flat, the bottom of each part is a good choice for the first datum (A). However, I am currently struggling with how to define good features for the other datum(s) which match my design intent (without needing to apply overly strict tolerances). What would be your recommendations for a defining good datums?