Need help with javascript allowing manual override of a field copied from another field

Clearly I am not not a javascript developer - i look for solutions and copy / modify as I can. 

I create fillable forms for a non-profit and have a requirement to copy a calculated field into another filed, as well as allow the copied field to be manually overriden if necessary - without changing the the value in the source field.

I've see a number of posts with requests for similar situations, but haven't been able to duplicate a viable solution - can anyone help?