Ratio between stone diameter and nominal diameter

For describing the relation between the grainsize and the nominal diameter (d50 vs. dn50) usually a factor 0.84 is used. This factor has been derived by Laan [1981] "De relatie tussen vorm en gewicht van breuksteen (the relation between shape and weight of pieces of rock), report MAW-R-81079". However, this original report cannot be retrieved...