As was demonstrated by Gy empirically, then elucidated by Francois-Bongarcon, only the P95 size of comminuted material (i.e. the size of a screen rejecting 5% of the material in mass) is relevant to sampling theory variance formulas. In practice, when establishing sampling nomograms or using historic data for heterogeneity testing, the P95 is not always known and the material is not always preserved for P95 determinations by screening. A non-theoretical study of the experimental material previously used and published by Gy had suggested some unpublished, rule-of-thumb, order-of-magnitude correspondence between other Pxx sizes (e.g. P100, P80, P75, etc.) and P95, to be used as a last resort when no better determination is possible. This correspondence surprisingly followed an arithmetic progression. In this paper, various types of size distributions were researched and most of these rule-of-thumb formulas were reasonably confirmed, except for the correspondence between P95 and P100 (maximum size), which was therefore updated. The reasons for the arithmetic progression and its exception for P100 also became clearer in the process.