I can give you some peace about your scale.
The conversion between grain weight and gram weight is 0.065.
I believe a penny in the USA (made after 1983) weighs 2.5 grams.
So if you put one penny on your scale, measure the grains and multiply by 0.065 you should get 2.5 grams.
You might try it again with 2 pennies to see you get 5 grams.
In short, grains or grams doesn't matter. As long as you reduce from what you started with.
Please note, no scale is perfect. But you do not need to be perfect. Just have a consistent reduction. Worrying about being perfect generates anxiety and anxiety makes withdrawal 10 times worse.