The tests i did was heavy wall and always 6" and above. A 5mm gap was needed to get the correct pen on the flat side, If you used a 2.5 it would take you for ever because of the obvious size and the pausing on the flat side, You would do well to get an inch out of it, loads of stops and starts. If you set a smaller gap to cope with the 2.5 rod, you wouldn't get the correct pen on the flat side.
5mm gap 1mm landing edge, dc neg 3.25 about 120 amps
I may have gone with a 2.5 on 2", but then i never tested on anything under 6" heavy wall, 6GR with a restriction disc