For the specific NMax calc previous cropping doesn't really come into it. So WW for example, your starting point is 220kg/ha. But you can add an additional 20kg/ha for shallow soil types, an additional 20kg/ha for for every tonne the expected yield exceeds the standard yield (8t/ha) and another 40kg/ha for milling wheat varieties.it depends on yield expectation, soil type, previous crop and markets (milling etc)...............
you need to use Planet really to be sure you are correct
But as I've said, you are not required to do the NMax calc. Just ensure you don't exceed the limit. Where previous cropping does come in is in the calculation of how much N your fields need. If you apply N of any sort, you must have done this calculation if you're in an NVZ.