webman1,
Unfortunately I can't offer much in the way of insight from what I can see.

I can tell you that because the suburb field is an open text box whatever your customer enters there, it will have to match exactly whatever you set up for the local tax entry.
ie- if you have a local tax set up to match for "Hamilton County" in the suburb field then that's exactly what they will have to enter in order for it to match.

Its kind of a risky thing.

LadyHLG