Outdoor DB and indoor WB

[tenletters]

I understand you need them for calculating superheat, but what's the relationship between the two? I've always just used them without questioning the relation of the two values.

[Saturatedpsi]

There's no relationship between the two, relative to determining target superheat. The need for the two values is necessary to define the pressure differential across metering device. The differential is essentially what determines how much liquid the metering device delivers to the evaporator coil. Head pressures follow outdoor temps...higher OD's, higher head pressure. If the head pressure increases, the differential will increase, so the rate of metering will increase.

Evaporator pressures also follow the "heat load" on the coil. If the indoor air temp increases, so will the pressure. Since the humidity level is also part of the load, it has to be considered for superheat calculations. Wet bulb temps take humidity into account, so the WB and OD temps define (sort of) the pressure differential "relationship" across the metering device. 

But of course, the equipment Mfg's have figured all that out some way or another, and just provide us with the target superheat chart.