Subreport value in report at detail level

I need to use a value in a subreport in the main report at the detail level. I have used the example that is all over the internet:
//@SubFormula
//Stores the grand total of the
//{Orders.Order Amount} field
//in a currency variable called ‘myTotal’

WhilePrintingRecords;
Shared CurrencyVar myTotal := Sum ({Orders.Order Amount})

And the corresponding code for the main report.
I need this at the detail level. Not all records will have a value in the field I am trying to pull. Once it has found the first record with a value in that field, it repeats it for every record from then on, even when there is no value.
How do I make it so it stops this and only pulls in when there is a value?
Thanks
Steve