Customize compound field

Hello, great job with you plugin.

I want to customize the plugin to work like this page: https://www.calculator.net/investment-calculator.html?ctype=endamount&ctargetamountv=1000000&cstartingprinciplev=5000&cyearsv=30&cinterestratev=8&ccompound=annually&ccontributeamountv=100&cadditionat1=end&ciadditionat1=monthly&printit=0&x=61&y=23

Your plugin works fine with “Compound” = “Monthly”.
But when “Compound” it’s Annually, Semiannually or Quarterly the value it’s different.

Please can you explain me witch file I must modify or how I can solve this?

Thanks in advance