Hi there,

The GHL ORP probe is going to be more sensitive to changes made in the aquarium and will therefore show the outcome sooner than the handheld instrument. If you have already calibrated multiple times and let the probe sit in the 220mV fluid and saw a result of 220mV, this verifies that the probe is working correctly. The behavior of ORP is a tricky concept to understand as there are many variables which can alter the end result; UV bulb age, supplements added, dosing schedules, etc. From your chart it seems 500 - 600mV is the norm for your tank.

ORP is commonly used as a general indicator for when a significant change occurs in the aquarium. There is no "recommended" range as each tank will have its own "normal" range. In your case, it is 500 - 600mV.
How does the tank inhabitants look to you? If they are all healthy, then this level does not bother them.