Flexigauge by ACL
Products
Plastigauge, Flexigauge
With practice, Plastigauge / Flexigauge readings can be accurate within about .0002 inch. However, the accuracy of these measurements are low compared to more expensive tools.
It's important to note that a difference of .0002 to .0004 inches may not seem significant, but it actually represents a 9- to 18-percent difference in accuracy.
If you don't know how to use an expensive tool correctly, you may encounter the same issues. In such cases, Plastigauge/Flexigauge, which have been utilized for a long time, could be a better alternative.
Regardless, plasti / flexigauge is the obvious option to have at home in the garage to check deviations with.