Interesting question - I have a balance doing something similar - an ohaus traveller that is needs 200g on it to get to zero!
wont even pass startup without it. I have kept it as a curiosity and in the vague hope I may come up with a solutions someday.
If only it were that simple - I have the manuals and the other 5 calibrated beautifully. This one needs 200g or fails at testing when you turn it on. I n both cases I suspect someone overloaded the balance by a large amount somewhere in the past. If there is a slater manual you should be able to calibrate, mine i think needs something replacing or straitening.
This place has some salter manuals if you know which it is: