David, your flatness PDF file is incorrect, you are measuring Parallelism in this example. True Position is correct, you are measuring Parallelism if you measure the top of the piece. You would use the Height Gauge to measure the underside of the part with the part on the surface plate with 3 stands to measure Flatness.
The easiest method for measuring
flatness is with a height gage positioned on a surface plate. Note that you must setup the part on three stands and find the minimum variation while adjusting them, just placing the part on the surface plate and using a dial indicator to find TIR on the opposite side of the part measures parallelism.
Mike