What's the datatype of your variables, {totalamount}, {netamountnotax}?
Also, {totalamount}-({totalamount}-{netamountnotax}) cancels-out to just {netamountnotax}, regardless of what {totalamount} is:
5-(5-4)=4
If you're trying to get just the tax amount, you'll just need:
{totalamount}-{netamountnotax}

I feel like if they're decimal/currency, you need to take the single quotes away from '0.00'.
If the environment requires strict types, you may have to cast 0 to decimal/currency, whatever your other variables are.

Hope this helps.