Make the field admin only, use the form publish shortcode to exclude that field ID, or hide it with CSS. Do nothing for the email if you are using the default message. It will get all the field labels and values and push through the email action. As for the subsequent page, build a view and where you want to show the calculation value put [get param=calc]. Then the form submit action will change to redirect to URL, then put the page where you have the view posted, followed by a ?calc=[field id here]. There may be more to the view depending on how you build it, but that is the basics.