When to use it
Start with the free diagnostic at /ecom-profit-vat-calc/diagnostic/. It gives you a concrete result first, then points to the paid product only if the next step is useful.
A UK Etsy or Amazon seller can model one product, see the marketplace fees and UK VAT-on-fees line separately, and compare the same sale before and after VAT registration using editable assumptions. They move from a rough margin spreadsheet to a reviewable, client-side pricing model they can check against their Etsy Payment account, Amazon Seller Central fee schedule, GOV.UK VAT rates and their accountant's advice. Publish the free diagnostic, list the product in relevant directories, then seed one value-first...