How to tell if a chain should be replaced with park chain checker when it doesn't fit?
As Argenti Apparatus indicated the "go" / "no go" wear gauge such as the CC-3.2 indicates that the elongation has not yet reached 0.5%.  If you want some sort of measurement below that (i.e., 0% to 0.5%) you will need to use a different gauge such as the CC-2, measure the inter rivet difference via a ruler, or use a pair of Vernier calipers to measure the roller to roller difference then convert it to a percent stretch.

Using Vernier Calipers

My preference is for the Vernier caliper approach as I could it to give very reliable and repeatable measurements if done right.  Methodology can be found in this answer.


Example of an inner link measurement (10 links apart)

I use the lookup table below to convert an inter-roller distance 10 links apart to percent stretch (calibration methods can be found here)



Predicting Service Date

I then periodically take a few measurements to predict service dates using a regression approach, such as the example below for a KMC X11-SL chain.  As you can see wear rate is fairly linear so extrapolating outside observed data range isn't too problematic (assuming we use the chain under similar conditions).  In this case, the regression model predicts it will be time to change the chain at about 3,250 km.   If you want to get even more complicated it is fairly straight forward to add confidence intervals to the prediction, then check again at the lower confidence bound.