I used a 4 mm ortho on a 740mm F 4.5 newtonian, ONCE, about 25 years ago.
Thats a mag of 825 ish.
I had a stable air cell go overhead and spent over an hour frolicking around the Tarantula. It was beyond words;; never ever forget how much of a WOW hour that was.
I reckon top mag since then would be around 300X... Still pretty amazing at times