How to set up SX to a loan account?

[Sergey Okhapkin]

The current debit/credit formulas in sheduled transactions editor work
OK only if no extra payments are made to the account. With extra
payments involved the calculations of interest and transfer to principal
values are incorrect. The formulas created by loan druid are:

Payment: pmt( rate / 12 : 360 : loan_amount : 0 : 0 )
To Interest:    ipmt( rate / 12 : i : 360 : loan_amount : 0 : 0 )
To Principal: ppmt( rate / 12 : i : 360 : loan_amount : 0 : 0 )

Interest and principal formulas are incorrect, these amounts depend on
loan_amount AND current account balance. The interest and principal
should be calculated in a different way:

To Interest = current_balance * rate / 12
Principal = Payment - Interest.

Where current_balance is the loan account balance before payment. I'd
like to modify the formulas in scheduled transactions editor, but... How
to specify current account balance there?

Sergey Okhapkin
Somerset, NJ