You can specify a monthly amount in line 5 and then press the TAB key.
To change the payment amount at a future date, note the line number of the last payment at the original amount, close the P.I. window,
enter the line number at the prompt, then press the TAB key.
To enter, change, or remove the default interest rate and final closure date: Enter defaults.
For a payback schedule of every other week click mortgagebw.
For a monthly schedule of constant 30 days, 10.5 hours (365.25 / 12) try Mortgage Test.
Theory behind this page.