Hi,
The tare weight is usually assumed to change with time, but i dont really know
what accuracy you are after either. Because of this, it is usually done with a
digital method where one reading is taken with no object placed on the scale
and another one after the object is placed on the scale. Of course the first
reading is then subtracted from the second to get the actual weight of the
object.
What else you should tell us is how you are actually converting the signals to
a digital readout, assuming that is how you are doing it. Depending on how
you are doing it (microcontroller, discreet logic, etc.) a different technique
will apply. It's simple to understand that for example if you are using a micro
then software would be the best solution, but if discreet logic then you may
have to add a few ic packages.
So, perhaps you can explain how you are actually converting the analog signals
to a digital readout for the existing design that you have now.