Based on what you're looking for (dynamically updating calcs), I would say for most people a spreadsheet fits the bill (Excel, Google Sheets, Calc, Numbers, etc.)
The tool you linked to has an intuitive UI and is useful for simple calculations. I can
see it being useful for every day use for folks who aren't that savvy with computers or math.
Spreadsheets however are a more much general tool (and only marginally more complex) that will let you do those calculations, but scale those calculations to much more complex models with almost no added complexity.
I have seen some really mindboggling things done with spreadsheets (and have done such things myself). But I can as effortlessly whip up the calculations on the demo page in almost the same amount of time.
I would say for most people, it would be a better investment of time to learn how to use a spreadsheet.
