Something I sent privately, but for the record...
FGMS servers have recently upgraded to versions capable of reducing/extending the 100-NM limit. This allows for radar clients to control sectors of different sizes, which is good, but makes any assumptions on client visibility ranges impossible.
Consequently, I think that it deprecates your choice of hard-coding a maximum distance between ATCs (currently 180 NM). I also think that it is not needed anyway, thus that the solution is fairly simple: remove the limitation completely.
Say A hands X over to B...
Please correct me if I am wrong, but to accept handover, the current implementation enforces:
1. dist(A, B) < 180
2. both A and B declare X as "in range"
Neither condition implies the other. So even if (1) is true, (2) can still be broken and the handover rejected. Why exactly did we need condition (1)? What would be the problem of removing it?