OK, I'm stuck on stupid here. This is the code behind the subform field (I did try several different variations):
=Val(DSum("Nz(Debit,0)-Nz(Credit,0)","qryCheckRegister","[Account] = '" & [Forms]![frmBankAccounts]![Account] & "'" And "ID<=" & [ID]))
To recap, frmBankAccounts is the main form...