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Back focus and change of focus are different things. I can't comment on the particular reducer you are using as I haven't got an edge scope or reducer, however back focus is referring to the distance between the reducer and camera sensor, not the setting of the focus knob. about 20 turns is probably similar to the change in focal plane with my reducer/corrector in or out of the train, but as mu scope does not have a built in corrector I can't compare any back focus difference meaningfully.
If it is supposed to have the same back focus as the bare scope, given an edge HD has a corrector built in to the baffle tube what it means there is if your corrector to camera spacing is right, you should be able to simply put the reducer in between the rear cell of the scope (Which has the corrector in it) and your imaging train, and if your field correction was good before, it should still be good after. The only other alternative would be if they mean that the back focus between the rear cell of the scope and the sensor stays the same with or without the reducer, but I really doubt that.
Another possibility (Again, without ever using an edge with or without reducer) is that your spacing was not correct to begin with and adding the reducer has made aberrations apparent that were off edges of the chip before.
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