You need a scale which can measure a thousandth of a gram. Many digital scales only measure hundredths of a gram .01 which is 0.154 grains. Hundredths scales don't have the resolution to do .1 of a grain.
If you google or search eblag for .001 scale you will find some, but of course vouching for a good one may prove difficult.
I just had a quick look and 253240120754 looks interesting. Its resolution is .005 grams which is 0.077 grain.
There are cheaper ones for the jewellery trade.