As stated previously, the numbers on the filters is arbitrary, however unfiltered Ilford MG is supposed to be the equivalent of grade 2. You can try a grade 2 filter and see how close to your results are to your original unfiltered prints, then adjust up or down accordingly. I like to use a filter to keep my exposure times longer to allow for any B&D that may be needed. In theory, you should have one contrast grade in mind and shoot for that, sometimes circumstances prohibit that so we have other contrast grades to choose from to adjust the image. You can either use graded paper as we all did in the old days, or use MG paper and change the filter.