I use the largest deep well socket that I have that will fit inside the steerer tube to seat the star nut. I suppose you could center it in the socket using a long bolt. That would help allignment when installing the cap. However, I have not had any problems just using the socket.

I agree with edthesped about leaving the steerer tube uncut until fit is dialed in. Heck, I still have not got around to cutting my LHT steering tube, and I finished the build over 3 years ago.