May be wrong but
But jsb PREMIUMS are quoted as plus or minus 0.001 in grammes from 0.547 grammes so a rounded up converted conversion would give you 8.42 to 8.45 grain. Now other people have questioned the weight range quoted but would they back their scales against jsb factory scales.Sorry if calculations are incorrect, but given in good faith i.e our take on what jsb are claiming.
At 55 yards indoors jsb exact 8.2 to 8.6 you are looking at 2mm this is real world,do it every time we need new batch when we bother to check use works scales cheapest start at 300plus . Chloe and Alan