Which rpr? First gen is 1.4 minimum, second gen can get away with a step lower since the handguard isnt the same height as the rail.
Cant tell you what the objective outside diameter is since nikon doesnt have adequate specs on their sight but it should be somewhere around 56mm/ 2.2 inches if its anything like other 50 mm objective scopes. half that is 1.2, thats what youll need to make up plus caps.
Edit: if you dig through the support and find a manual and dig through tat its 2.4" outside diameter.