As an example, if the task is started on 10/16 and ends on 10/21, technically on 18th the % complete should be ~50%. However, if you want the % complete from 17th, it should say ~25%

This can be achieved used the following User Fields -
SYN_Relative % Complete = FLOAT(    (DATA_DATE  - UFV("SYN_Relative Date")) /86400)/ FLOAT(( PROPERTY("Planned Finish") - UFV("SYN_Relative Date")  ) /86400)    *100    )