Can you make a calculated field user-editable?

Yes you can. See my comment in the following thread: http://acrobatusers.com/forum/javascript/display-value-was-input-rather-calculated-value

Thanks George. I couldn't understand how to apply this my form. I have added a combo box instead and the calculated field acts on the value of that combo box (either tax or no tax).
However,
1) Changing the combo box doesn't automatically refresh the calculated field so it could be confusing for the user. Can I have the field "refreshed" when the combo box changes. I'm sure I must be able to do this with java or something.

2) would rather have an option box instead of a combo box but I don't seem to see a way to use 2 check or radio buttons instead of a combo box. Is there?

Thanks again.

You can do it with either a combo box or by using check/radio buttons, you just have to set it up correctly. It could be that the field calculation order is not correct. It would be easier to provide more guidance if I could see a sample form. Can you post one somewhere, like Acrobat.com?

I can't post it, it's a labour union member only form, too bad. However, I did change the combo box to radio buttons and clicking on them DID make the form change (calculate), unlike the combo box which seemed to sit there till you clicked away from it. I might have to go back to a combo box though as I now have more than 2 options, which is cumbersome using radio buttons. That's a drag. Wouldn't a simple Refresh command somewhere do the job?

You can set up a combo box to commit the selected value immediately. It's one of the options on the Options tab.

Yes, I had a combo box set up, working fine, but when you changed the options, the calculation doesn't automatically change until you tab or click away from the field, that's the problem. The radio button, on the other hand, does make the calcs change on click.

It won't be a problem with a combo box if you select the option I mentioned in #5 above, assuming the field calculation order is correct.