Your yardage estimate is as accurate as we can reasonably make it, but there are some factors that will increase its accuracy! A scale that measures weights in increments of whole grams can contribute to over- or under-estimating yardage requirements.
For instance, a scale that rounds to the full gram will display 20g for a swatch that weighs anything from 19.5g to 20.4g. This leaves a lot of room for error when it’s multiplied at a large scale.
The best way to get a good yardage estimate is to use a scale that rounds to the tenth of the gram; if that’s not possible, take your yardage estimate with a grain of salt or use one of these techniques to estimate your yardage.