First, let me add that generally, you do not want to save calculated amounts for the basic reason that they need to be recalculated whenever the data changes. That having been said..
On one of the properties of the subform, like the On Close, put code similar to the following
[HdrCalcField] = Forms![sfrmCalcAmt]![CalcField
You may have to play with the syntax since I do not know you form/subform setup. If you have difficulty, try using the expression builder to see the syntax and then put it into your code.