A feeler gauge is a tightly controlled plate with a known thickness.
The technique to using a feeler gauge is the go/no-go technique. First a small (thin) feeler gauge is entered into the gap being measured. Then a large (thick) feeler gauge is verified that it cannot enter that gap.
Now we know that the gap in question has a value between the two thicknesses of the feeler gauges.
Problems with Feeler gauges
This technique is highly error prone when applied to setting up a seamer. First, the values are extremely rough and inaccurate. In many cases the failure to insert the feeler gauge can be caused by other factors (not just the gap between the roll and the chuck).
In most cases, when locking down the value set by the feeler gauge, the actual value will change, which means that feeler gauge setup is mostly a guessing game.
Alternatives to Feeler gauges
The main alternative to feeler gauges is the Clearance gauge which is used to provide a live image of the gap between the roll and chuck and provides a measurable and repeatable way to set up the seamer. The Clearance Gauge can be left in the seamer after the roll has been locked to see which position it goes to when locked.