I want to add custom CSS statements to style my calculator. Where should I add them?

Custom CSS code should be added to your theme's Custom / Additional CSS field.

If you use WP Bakery Page Builder (former Visual Composer) you should click on Page Settings button in the upper right corner of WP Bakery Page Builder's interface on relevant Page Edit Form:

Alternatively, you can use third party plugin, e.g. Simple Custom CSS.