Use the get summary function. Specify the same break field in this function as you have for the sub summary layout part. This will then return the same value as the value of the summary field placed in the sub summary layout part and you can now use that value in a calculation.
I created a Summary function as you indicated, however, it does not appear in Browse Mode. Am I doing something wrong?
Make sure of the following details:
The field should beof type calculation, don't use a number field with an auto-entered calculation
The "break" field in the get summary function is the same exact field that you specified as the "sorted by" field in your sub summary part
The records must be sorted in an order that includes the break field.