The easiest way is connect rover to your NTRIP and survey over a known monument.

The coordinates you get out of the rover should be in the same datum provided by NTRIP and line up with coordinates for the monument (assuming monument and ntrip use the same datum).

Had to google for while, not easy finding numbers.

According to Trimble:

*### Base station coordinate accuracy*

*The base station coordinates should be known to within 10 m in the WGS-84 datum for optimal system operation. Incorrect or inaccurate base station coordinates degrade the rover position solution. It is estimated that every 10 m of error in the base station coordinates introduces one part per million error in the baseline vector. This means that if the base station coordinates have a height error of 50 m, and the baseline vector is 10 km, then the additional error in the rover location is approximately 5 cm, in addition to the typical specified error. One second of latitude represents approximately 31 m on the earth surface; therefore, a latitude error of 0.3 seconds equals a 10 m error on the earth’s surface. The same part per million errors apply to inaccuracies of the base station’s latitude and longitude coordinates.*

So shifting base around 50cm in either direction to make it line up the datum for your area doesnt affect the accuracy. Which make sense cus using average position on base might get alot more then 50cm of the true position.