Override calculated tax line on AP invoice

 I posted this in the eBiz Tax community a couple of days ago but have not received a reply.  I'm wondering if anyone in the Payables community can help with this. The AP user guide indicates three settings for allowing the override of a calculated tax line:

Event Class

Configuration Owner

Tax - Controls and Defaults

I've checked all three and they are set to allow override for calculated tax lines.  However, we have not been able to change the amount or account on a tax line.  The only way I've been able to get an override accomplished is to set the tax control amount at the header field and then calculate again.  However, this doesn't seem the logical way to override a tax line.  It could be an account change or an amount change and this workaround only results in an amount change.  Is there some other setting that I am missing?