What is the standard for temp. difference between supply and return air?
How good can it be?
heating or cooling?
cooling 15-20 deg.
heating, whatever the data plate states for a heat rise, shoot for the middle of that range.
if a heat pump as opposed to fossil fuel, then, as a "rule of thumb" which doesn't mean much, in the heat mode without any aux heat running, at 32 deg o.a., the split should be around 20 deg. for every 3 deg. higher, add 1 deg of split temps.
i.e. 35*=21 split.
this is not a perfect example, but only a reference.
a good temp. is 15 to 20. Why because if it goes to 25 to 30 the evap. coils will start to get frozen.
Is your heat pump, in the heat or cool mode?
In heat it will vary by the ouotdoor temp.