Q:

How to tell if a chain should be replaced with park chain checker when it doesn't fit?

I have a park chain checker CC-3.2 which has two sides to indicate when a chain reaches .5% and .75% wear. 
However, since I often have some filthy chains due to off-road riding and daily commuting and multiple bikes, I lose track of which ones are worn a little and which ones are worn a lot, Especially after a winter of commuting in salty east coast slush, fine sandy spray from the rainy season, and dirt/sandy stuff flying off my tires from rail trails and singletrack. I've definitely had chains worn well over 1%. 
So, Other than performance issues, when neither the .5% and .75% sides of the chain checker fit, how can I tell whether a chain is relatively new (and not well worn) or whether it's really worn (probably past 1%) just using my super basic chain checker? 
Does it fit differently or is there anything specific to look for while it's sitting on the chain?
To rephrase: If I have two chains, one with say 0.25% wear and one with 1.25% wear and neither chain allows the indicators on the Park CC-3.2 tool to drop between links, how do I tell the difference? Can I do that by just looking at the park chain checker?

A:

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.

A:

If neither size of the chain checker tool will fit between the rollers, the chain is less than 0.5% worn. The Park Tool is not supposed to have more resolution than that.
I personally keep a ride log (relatively easy with Strava and Wahoo) so I know how many miles my bikes have done. 

A:

How to identify extreme chain wear on a Park CC-3.2 (or similar tool) with only 0.75 side:

Chain is < 0.75% wear, it will hit on the outer edge of the tool and the indicator will not drop between rollers.
Chain is >= 0.75% wear and probably chain is < %1, it will drop between the rollers.
Chain is probably > 1% wear and worn more than a few percent, likely > 1% wear, it will hit the inner edge of the tool and the indicator will not drop between the rollers.

If a chain is very worn and the measurement gauge hits chain links on the inside edge of the gauge as opposed to the outside edge of the gauge, this can indicate that the chain is well past the .75% wear. This technique can be used to identify whether a chain is heavily worn when you only have a "GO / NO GO" chain checker.
You can see an example of a heavily worn chain showing this condition below (See inner edge contact instead of outer edge contact).

Image Source: Dalemorrison66 on Instagram

