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If the specification needed does indeed require a range of 0.100", an immediate measurement remedy for the resolution issue would be to use a 6" steel scale in addition to your current tape measure. The standard rules discriminate to 1/64" (0.0156" -which exceeds the recommended 0.010") but the tape measure can be locked and then use of the 6" ruler can help better estimate the variation around your nominal.
This may improve your GR&R and if it doesn't there may still be an opportunity with the originating measurement datum as previously mentioned.
This may improve your GR&R and if it doesn't there may still be an opportunity with the originating measurement datum as previously mentioned.