When using Layout Tester, rather than specify a percentage of traffic that still goes to Ad Tester, why not allow your original website template (i.e. the Ad Tester scenario) to compete for every single impression much like the Layout Tester templates do?

I'm assuming that when you set a % of visitors to see your Ad Tester template, the system doesn't take any of the many factors into account such as the source of traffic, the country the visitor is in, the landing page, whether they are new/repeat visitors, what screen size they are viewing on, etc? It just arbitrarily sends that % to your Ad Tester. In which case, Ad Tester is always at a disadvantage compared to the other templates that may be shown to specific visitors who match the right profile.

Instead, why don't you allow the system to treat the original template (i.e. Ad Tester) as it would any other template and test it with samples of visitors to eventually optimise who sees it. If it is only shown 2% of the time, fine, but if it is shown 40% of the time, that's fine too.