OK Don wrote:
The feel part of using a feeler gauge is to have some resistance when pulling the gauge through the gap, but not so much that it's changing the gap -- no resistance means that the gap is too wide.
One thing I've found helpful, when I'm in doubt, is to do a go/no-go test. Try a gauge the next size up from the spec. It should fail to go through. The proper size gauge should go through with a slight drag, and the next smallest gauge should fall through with no resistance.
It's a feel sort of thing...with the proper adjustment, the gauge will go through, but you'll feel it slide against the surface on each side. Be careful not to tilt or twist the gauge, or you'll get a false reading.
