I'm a full stack applications developer. I specialize in business applications.
I've done three very similar projects:
1) a javascript auto-loan calculator
2) a mortgage reporting tool that uses [login to view URL] and an API with 20 years of loan history information
3) an FMLA cost assessment calculator
I can build what you are looking for.
As all three of the above mentioned projects were for private business use, there are no public links to those tools. However, if you'd like to see them to get an idea of my ability, please let me know and I can share temporary links to my development environment.
I do have a quick question does the existing page already use jquery and bootstrap?