Calibration Standards

The simple answer is that you want to keep a 4:1 ratio. So a +/-0.01875% of reading instrument would be great.

This 4:1 ratio is that a rule of thumb,a good practice or is it a standard.

When you talk Calibration and Standards, you really raising NIST Traceable Standards. The Boulder Colorado outfit provides Standards and Services to calibrate agains national standards. Then there are outfits who provide calibration services against their second tier standards, and so on. If you want it certifiable, or to ISO standards, that is the way to go.

You may decide to have your equipment recalibrated periodically by such a service, or have a better standard at your facility, where you yourself can perform periodic recalibration. Neither is inexpensive.

Get well acquainted with it, before making decisions.

NIST suggest 4:1 but as a minimum your equipment should be 0.375%

It appears to be a misprint. A test equipment normally needs be more precise, than the stated limits of the equipment to be tested. In this case 4x more.

The other point is that, if you want the results certifiable, it needs to be traceable to NIST standards. Hence, the multitier approach, also the need of periodic recertifications.

Do you need the .075% accuracy, if not you can calibrate the transmitter to the accuracy your standards will allow.