I've tested your form and the result have only two decimal digits with all combinations of values used.
About hide the calculated field until the calculate button be pressed. I suggest assign a classname to the calculated field through the "Add Css Layout Keywords" attribute, for example myclass, and then paste the following class definition in any of CSS files of your website:
Finally in the equation uses the snippet of code:
In this case, the equation should be defined in the format: