It's not trivial to import the HTML into a VS project. Wasn't trying to say that. But it can be done. Mainly, you have to stop VS from getting in the way. Anyway, I do agree that this project seems to call for a browser-based, js-based solution.
As for user-friendly: What I meant was, suppose somebody specified the number of square feet but then neglected to specify the depth? You C# code would simply give no answer at all. It would be better to have it say something like "You need to select the depth as well". That's all. No big deal, an 80% of people would have no problem without it. But trust me, that other 20% would drive you nuts with phone calls about how they can't make your web page work.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.