Entering formulas in scheduled transactions

Daniel Hannum dhannum at magicdan.net
Tue Sep 9 03:00:21 CDT 2003


On Mon, 2003-09-08 at 21:39, Derek Atkins wrote:
> > Unfortunately, the druid formulas don't always reflect reality, and can
> > even be way off sometimes.  For me, even a penny off is unacceptable--I
> > require exactness in my financial picture.  If there were a way to
> > specify a custom formula for anal retentives like myself, SX would
> > probably be more attractive WRT entering loans.
> > 
> > My loan holders think that penny matters; guess I ought to, too...
> 
> I've noticed (well, other users have noticed and I've helped track
> down) that some of this is due to truncation (generally of the
> interest rate).  Some banks seem to truncate the rate(s) when
> computing the payments.  You might want to play around and see what
> happens if you truncate your periodic rate to, say, 4 significant
> digits.  Does that better match what the bank says?
> 
> Perhaps we should let the user provide a "level of truncation" in
> order to get the numbers to match properly?

I think all we have to do is allow the user to specify their own
payment, in case it is different from what gnucash calculates. That's
not actually hard to implement. I had part of it done before... I need
to get the gnucash development environment back up so I can finish it.

The hard one is properly dealing with paying down the debt. The
sched-xaction is generated once and it just blindly figures out
principal and interest payments based on number of periods, initial
balance, rate, etc. If you pay it down, it WILL ignore you and you'll
have confusion at the end of the loan, when the sched-xaction continues
to deduct. You'll have to manually fix it all up.

I guess the simplest solution would be to have it recompute at each
sched-xaction interval: take the current balance at this moment,
multiply by the interval rate and that's the interest. The rest is
principal. If the balance is now 0, kill the sched-xaction.

But then there are fuzzy issues such as, on what is interest calculated?
Balance on the 1st? Average balance over the period? Balance just before
this payment? There is probably a convention to this that I am not aware
of.

And then you'd have to deal with Jordan's original problem: compounding
semi-annually... I'll have to think about this more, but I think I'm
onto something... It's 2am local time... zzz...

dan



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