You need a scale that is precise enough to do so—there’s no way around it. For an amount like 0.46 grams, you would need a scale accurate to 0.01g.
If your scale is only accurate to 1g, you will not be able to weigh out amounts less than 1g, or between grams. Since your scale can only register whole grams, that is all you can reliably measure.
For that reason, I highly recommend investing in a scale accurate to at least 0.1g, if not 0.01g. I only use my 1g increment scale for making 1kg+ batches of soap; all my cosmetic formulating is done with a 0.1g and 0.01 scale.
That said—the amount of precision required in a formulation varies with the formulation, the batch size, and the ingredients. If a preservative is called for at 0.1g, and water is called for at 89.1g, that 0.1g is a lot more important for the preservative than the water! You could round the water down to 89g and it likely wouldn’t have any noticeable impact on the end product, but that wouldn’t be the case with the preservative.
It’s pretty common for spreadsheets and percentages to crank out some pretty long, ultra-precise numbers (3.94329g, anyone?). For our purposes, you can feel pretty comfortable rounding to one or two decimal points. For ingredients that are incredibly potent and only ever used at amounts well below 0.1% (water-soluble pigments, pH adjusting ingredients, some actives), it can be a good idea to pre-dilute those ingredients to create a stock, and use that in formulations (I do this with hyaluronic acid).
Posted in: Measurements