Optimizing the amount of solder paste transferred during component dipping for Package-on-Package assembly should be a goal for your PoP process. While the increase in solder generally improves solder joint reliability, too much solder paste may cause bridging. It is important to find the fine line between too much and not enough solder paste, and adjusting reservoir depth is one easy way to find that line.
Reservoir depth can be easily adjusted on some equipment, although simplistic dipping equipment may have a fixed reservoir. Assuming the flux reservoir is adjustable, the depth should initially be set to 1/3 the BGA’s standoff dimension. This may need to be further optimized, although it is a good starting point.