Any tool/app for tracking open line of credit from Family/Friend

[Jerry Villa]

I'm getting an open line of credit from a family member and wanted to give them interest on it. I'm not sure yet how much I will need as it will be used for any incidentals that are not covered by home depot credit card and other financing. It would be easy if it was just a full set amount loan of $XXXX.XX and just attach interest, there are loads of calculators online for that, but what if its an ever changing amount used, so maybe in the first 2mons we pull about a small amount and then later pull out more while paying it down. not sure what that is called and if there are any tools that can track interest on what is pulled out and at each stage of the balance. 

Thanks!