[GNC] Mortgage & Loan Repayment Assistant woes

[Chris Mitchell]

Hi all,

I can't seem to get my mortgage repayment scheduled transactions set up
correctly, even though I *think* my mortgage is pretty close to the
simplest possible case.

* It's a fixed interest rate;
* The payments are only principal & interest, no tax/escrow/etc;
* Every two weeks I make a blended payment with a known, fixed total
  amount;
* I have from my bank a schedule of payments so far this year,
  projected payments, balance after every payment, etc

On the Loan Details page, I select the mortgage account, enter the
current balance remaining in the "Amount" field, and enter the interest
rate (selecting "Interest rate" and type "Fixed" on the accompanying
dropdowns). For the start date I put in today's date, for both "Length"
and "Months remaining" I provide what Libreoffice Calc's "Months"
function tells me is the number of months between today and the "Ending
Date" listed on the mortgage paperwork from my bank.

On Loan Repayment Options, no boxes are checked off.

On Loan Repayment, I give the transaction a name and select the
appropriate accounts for where the payment comes from and where the
interest goes, then specify the frequency (every 2 weeks, on Thursday,
starting today). The "Amount" field is a non-editable display showing a
formula that I don't understand, namely: 
pmt( [MyInterestRate/100] / 12.00 : [MyMonthsRemaining].00 :
[MyPresentBalance] : 0 : 0 )

(Actual loan details redacted)

I recognize the values it asked me for, so that's good I guess. At no
point thus far do I see anything that I recognize as a place to enter
my *actual payment* amount, which strikes me as surprising.

Then I click "Next" and look at the list of generated payments. The
overall payment amount showing here is a little over three times my
actual payment amount, with the interest portion close to (but not
exactly) twice what it should be.

I assume Gnucash is doing the underlying math right, so the result
being so thoroughly wrong would seem to imply that I'm doing something
terribly wrong in the entry process. I've done it three times, double-
and triple-checked my values against the bank paperwork, and I'm
stumped.

Any help to get my scheduled transaction set up correctly without
having to go learn the whole "fundamental financial equation" business
(https://code.gnucash.org/docs/C/gnucash-guide/loans_calcs1.html) would
be gratefully welcomed.

Cheers!
 -Chris