AFM Voltage with pre cats removed

[selnic]

Have just removed pre cats from std Griffith 500, has anyone got the correct voltage for the AFM setting - at the moment it is still set at 1.8 Volts engine runs fine but on inspection of the (new)spark plugs after about 60 miles they seem quite sooted.RPi site which every one seems to use only give voltage for pre cat and cat model, should it now be somewere between the two?
Nick