If available, try to use the 6T with a fixed focal length lens. I would expect much better results. I sometimes used the 6T many years ago with the Nikkor 1.8/105mm. This combination worked very well. Today I am using another achromatic lens (Pentax T132) to increase the maximum magnification of my 2.8/165mm lens from about 1:7.7 to 1:3.7. Again, the results are very good.
5T and 6T diopters were killer on the Nikon AF 80-200/2.8, just like 3T and 4Ts were on the older 80-200/4.5--great results from each. Primes work fine but zooms are way more convenient for macro.
6T diopters have become pricey and hard to find after Nikon dumped them a few years ago. Lucky you, Galah.