Nice design!

Caveats - not all countries have the same vat rate for all things and sometimes items are VAT exempt.

Vat applies to currencies so decimals/commas should be present (or another local number convention).

I toyed with the idea of having no buttons because I don't like the ambiguity of what the entry field represents, so you can do this without the need for any buttons and the user could enter any variable and have the other values change accordingly. I was too lazy to get the fonts or spacing exactly right.

Click image for larger version. 

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Paul