I needed to update this thread and my image after some testing and adjusting. I wondered how my calculation using the Starizona adapter and the backfocus of 59.7 mm - as given by William Optics - could work.
The answer is simple: The pezval construction of the WO RedCat51 does not require a precise backfocus, because it's focuser works more like a camera lens and moves the optical train. You just need to be in a range of backfocus, which allows you to move the focuser enough forwards and backwards.
I found a backfocus of app. 54.8 mm to be an optimal length.
Using the 59.7 mm which are stated by WO, my focus point is exactly where the focuser is completely moved in. Adding filters moves the focus point out of reach.