I am trying to transfer our calculator that we developed for our old site into Sitefinity. However, once I transfer all the code into the html section of the content block, Sitefinity doesn't recognize "<%" and it instead of treating it like code, it treats it as a regular text and changes it to <%. Is there a solution to this?
The ASP.NET code that you're trying to use won't work in a content block for a number of reasons.
The correct way to accomplish what you are trying to do is to create a custom widget for your site. Here is the related section of the Sitefinity documentation:
User and Custom widgets
I hope that helps to point you in the right direction.