There is no link between the N-Tester BT reading and the level of nitrogen to apply for protein. The N-Tester BT is aimed at providing a recommendation for final nitrogen applications to achieve optimum yield.
What the N-Tester BT will do with continued monitoring is tell you how much nitrogen the crop is able to pick up and will help identify what is happening in the plant. The most important starting point for achieving milling protein spec is to get the nitrogen right for yield. Wheat crops remobilise nitrogen from the leaves to the grain during flowering, but this does not start until mid-flowering, therefore if, through weekly N-Tester BT reading, you identify levels of nitrogen in the plant falling prior to this time, it can help to identify shortages in nitrogen which will need to be rectified prior to applying the late-season nitrogen for protein. As this is likely to be beyond the GS45 cutoff, the absolute recommendation will not be relevant, however, something would be beneficial to ensure the crop is optimally fed.
If the nitrogen levels continue to increase or remain flat, then you can be more confident of optimal nitrogen rates, and therefore just go with the late nitrogen application at the standard timing.