Sussed out the arm positioning. Two arcs describing the distance from platter centre to arm pillar centre. One for FFSS and one for 774. both arms will use SME type bases (I'll make one for the FFSS). I've elongated the arm opening that would normally be used for the 774 as the FFSS is slightly shorter and will mount further forward. The position is the only one that allows both arms to be used using SME mount four bolt spacing and positioning and not have edges of the base plate overlapping the TT top plate. The arm counterweights may overlap the TT top plate a bit. Not worried about that. The plinth will be built to accomodate it.