The easiest and cheapest solution is to buy a powder measure. There is the question about how accurate the measure is, and since grains are a measure of weight and not volume, the only way to 'calibrate' the measure is to weigh the powder from various settings. This can only be done reliably with real black powder, as it is grain for grain the same number whether measure by volume or weight. Within a couple of percent anyway. The scale used to measure the weight should be a good balance scale as mentioned above.
But, that's a lot of effort for little gain except perhaps confidence. Just buy the powder measure and go with it.