Yes you should place your code in there. Check if player is normal or premium to be affected by different rates.
multiplyChance *= player.isPremium() ? Config.PREMIUM_DROP_RATE : Config.DROP_RATE;
Something like that for every case, drop, spoil e.t.c
Just don't open aother crappy interlude server! No I'm not flaming acis xD It's just because the client is outdated! Use one of the new clients and downgrade them to your target chronicle. This way you do not need to pay people for custom weapons, maps, clientfixes bcs win10+ or whatever.
Just take a newer client and customize it. The oldest client that actually could be used for this should be H5. For the new ones....depends on you which one you prefer, because of the interface chan