Can the Test Option for rates be allowed to float so you can edit the rates and retest without having to enter the data again?
It would be great to have a link to the tester directly from this list.
The rate checker could be more useful. For example the ! that indicates an error - how cool if that took you to the area where you needed to edit the rates.
The best thing to do is to open multiple browser tabs. That way you can test in one and make changes in the other.
You can also open rate tester in its own separate tab (instead of the modal) by middle clicking the link.