Honest opinion... I don't see a need for it in it's current format.
If I already know the vat rate, it's literally 2 seconds on a calculator to work out before/after vat (hell you even show the formula on the page). Anybody who uses excel to do their accounts with even a basic knowledge of excel can have a spreadsheet set up like you offer for free and those with more advanced skills will have even more complicated/linked sheets within the excel file.
What I wouldn't mind though is either a tool or a drop down/list of all the different vat levels for each country because trying to remember all of the 'exempt' or 'reduced' things can get tiring lol... Utilities is 5% for example if below a threshold in the UK (it's 33kWh electric per day so most of us on this forum would be 5% not 20%.) and there are others which are either lower than 20% or exempt.
It could be an API to be linked with accounting or estimating software. We deal with printers across the world, USA, China, Singapore, India, Italy, Germany etc. Be super handy to pull out their tax rates into an estimate. I presume.