Accuracy in %FS and %RD.

I have a field transmitter say a FT (with accuracy in %RD) whose signal goes to a signal isolator (with accuracy in % FS) and an indicator ( with accuracy in %FS) in panel. Generally to know the overall accuracy, we take square root of sum of squares of individual accuracies. Is this method applicable when one instrument in the loop is having accuracy in %RD ? If not how to calculate the overall accuracy?. More over in some instruments, accuracies are expressed in engineering units (like Radar level transmitter). When a loop contains such instrument, how to calculate overall accuracy?

If this enquiry is posted previously, please share me the link. Suggest any good book to read on this.
Accuracies of field devices and also of indicators, controllers... listed in technical documentation are intended under reference conditions in laboratories (T, P, humidity, no interference, ideal reflection for radars...) , according to specific norms such as DIN 61298-2 or others. In the process such accuracies can vary dramatically. The only way to calculate a reliable accuracy of a measuring loop is to perform a calibration with a reference master, more accurate than the instrument under test