Which is the better UX for a user to calculate VAT?

I created a universal VAT calculator. I thought it was user friendly, but one user on the talkgraphics forum refuted my opinion(

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.

And here is my version of the calculator's interface:

Which interface is better? Here you can try my version of calculator https://vatulator.com/