Considering I don’t use the term “visual carat weight” anywhere on my site, I think you’re asking in the wrong place. You should be asking other sites how they calculate it.
For every carat weight there is a distribution of spread (surface area). This is shown in something like my round diamond diameter chart. You could say that a particular 1ct diamond looks bigger or smaller than average for a 1ct diamond because you compare it to a large sample of other 1ct diamonds and see how it ranks. But I think saying that a 1ct diamond looks like a “visual carat weight of 1.07ct” gives a false sense of precision because 1.07ct itself has its own distribution and there’s no mention of what those ranges are and how these continuous distributions for all carat weights overlap.