Determine Tax Code for Country / Product Category

Wonder if anyone can help with this one.

All my tax code determination currently has entries similar to the following in customising:

I Domestic GB 511AA RQ5CLNT300 V2

I Domestic GB 511AB RQ5CLNT300 V9

I Domestic GB 511AD RQ5CLNT300 V9

I now need to add entries for other countries that we procure from, ie the US.

I've entered information similar to the above, ie as follows, but this still doesn't seem to work and I get the message "Unable to determine tax code"

A International US 511AB RQ5CLNT300 V9

We are not using TTE.

Has anyone got any ideas on this one ?

Thanks in advance.

Keith