mortgage payments: split between principal and interest?

[Matt Price]

perhaps a stupid question:  

I'm trying to schedule payments on our existing mortgage to my new
gnucash file.  I'd like to use the mortgage/loan druid but it doesn't
seem to be quite flexible enough for what i need.  

I've checked the actual amortization schedule on my bank website, and we
have 662 weekly payments left on a variable-rate mortgage with a
remaining principal of 213258.21 and a current rate of 2.15% (I don't
know how often the rate changes, though i think it's not more often than
quarterly).  the druid generates a payment formula that looks like this:
pmt( 0.02150 / 12.00 : 156.00 : 213259.21 : 0 : 0 )
but that ends up getting the amount wrong.  can anyone suggest an
improvement, or suggest where to look for more help?

thanks as always,
matt