sorta rule of thumb but certainly logical.
If the heat pump is to perform without the strips kicking in you could expect it to produce at least 12 to 18 degrees defferntial. I think anything less than 12 and the strips would not be far from kicking in.

I have always removed the cover of the t-stat so that I can see that the first bulb only is engaged while checking the heatpump temp, then raise to engage the second bulb and look for a significant increase.
The combined stages will produce the more unpredictable temp but 100 to 130 degrees should be expected.

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Consistency is a direct path to perfection.