What might be going on is that Rega uses a proprietary alignment/arm geometry. Their arms use, well for lack of a better term, "Rega alignment". Of the common 'universal' alignment methods, I -think- I remember reading that Stevenson is close to Rega. Close but not exactly the same. With a given cart, if the stylus point is further away or closer to the mounting nuts by some degree compared to Rega's carts, this combined with Stevenson being a bit different, could mean that a given cart simply will not align to Stevenson on a Rega arm while remaining square in the headshell.
I encountered something similar with an old arm from Hong Kong- it simply would not align to a known universal method and I had to research the original overhang spec for that arm, and once aligned using just the overhang (stylus distance beyond the center of the spindle)- the arm tracked wonderfully.
I think the above advice to start with 15mm overhang is good.... as I'm pretty sure that's Rega's spec. Its a good starting point.