Kingsley, although a formula per se is not provided, the graph on page 12 of this document might be useful for you - https://www.hort360.com.au/wordpress/wp-content/uploads/2020/06/Fertcare-Soil-Sampling-Guide.pdf
there are a couple of references that may be worth following up on, although I'm not familiar with either.
Kingsley, Murray's comment is correct. The more fundamental problems elate o the site size and variability, and to the purpose of sampling, e.g., to characterise the site or the variability, Paul.
Soil sampling depends indeed on many research questions, as mentioned by Murray Hart. I advise to look at an article on Researchgate.net: "Current Soil Sampling Methods - A Review" (May 2018) if your interest lies in agriculture.