I am assuming that you are using summary fields to get the min and max values. As you have discovered, using a summary field in a calculation results in the summary field returning a "grand summary" value. To get a sub summary value, you need to use the getSummary function. Specify the same "break" field in the getsummary function as you specify for the sub summary field's "when sorted by field".
One potential issue to avoid is that, unlike a sub summary layout part, the break field in GetSummary must be defined in the same table as the summary field.
PhilModJunk, you NAILED it!! I'm not sure I had a need before to use the GetSummary function, of if I did, it was years ago. As I was researching my solution, I found and included it in the formula with zero results, and it was as you said because it was not in the same table. moved the data in to the table and it worked perfectly.