Greetings!
Might I assume that you are using a strain gauge as your actual instrument? It should be an easy matter to do this; you can just use an ADD node to your data, and use a front panel knob to zero the tare. Alternatively, you can use a negative feedback loop to do the zeroing automatically.
Eric
Eric P. Nichols
P.O. Box 56235
North Pole, AK 99705