Even though InDesign allows you to create buttons with simple actions that are then converted into PDF form fields, it does not support calculations. I would add the fields and calculation scripts in Acrobat after you create the PDF file in InDesign. If you need to update your document, you can use the "Replace Pages" feature in Acrobat to get the new document content into your document with form fields:
Open the "old" document that has the form fields and calculations in it, then select "Replace Pages" (e.g. by right-clicking on a page thumbnail in the thumbnails pane and selecting the "Replace Pages..." menu item) and select to replace all your pages with the pages of your "new" (that is the updated document that just came out of InDesign). This gets a bit tricky if the number of pages that you have in both documents is different. You may have to duplicate a page in your "old" document to get to the correct number of pages. As long as the page with the form fields in the old and new documents are the same, you should be OK if the other pages contain random information. This will replace the information "behind" the form fields, and will leave the form fields in the location they were in the original document. You can then move the fields around and align them with the new content.
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