can't give you short answer on that, but i will try explain.how do I find that out?
on Sinerania, there are 30 Diamond Guilds this round
each map can have from 5 up to 8 guilds
if 5 or 6 guilds in a map then 1 goes down to platinum (assuming all have 1000LP)
if 7 or 8 - then 2 goes down (assuming all have 1000LP)
to that, on some rare cases, there could be "pull ups" from platinum into diamond when not enough guilds to make groups of 5~8 guilds.
makes sense ?