Setup of Ortofon SPU Royal GM II in Ikeda 407

I would love some feedback on how people set up their SPUs in an Ikeda 407. The Ikeda has a few different setup parameters. The main one is to use 307mm from pivot to spindle and then just position the cartridge at the 50MM point in the headshell (measured from stylus to end of headshell) to get it right. But this does not conform to any of the known null points and has me confused. The SPU is 51 MM in length and going beyond in length is much worse than going less in length. Anyone w. experience to chime in would be appreciated. Remember the SPU Royal GM has the cartridge permanently fixed in the headshell. The only adjustment I can make is in the positon of the tonearm base.