Ok it appears that I underestimated DH's income but I'm not how to work out how much we owe, as his income changed through the year cos of overtime paid.
For 1 month I overestimated about 10 grand, but for the other 11 months I underestimated $500.

Plus - for 1 month I had my income as 4,000 and for 11 months had it at 1,000 but I didn't earn anything - so will that cancel out the $500 I underestimated?

Not sure if they are the same or not.

Cheers!