The soft credit amount calculates after hitting the Enter key.

So to have the soft credit amount calculate out:
  1. Change the Amt/Pct column to Pct.
  2. Enter the percentage in the Amount column
  3. Click the Enter key, NOT the Save button.
  4. The option changes back to Amt, but the Amount column should now contain the calculated amount.
  5. Click Save and you are done.