I want amount, not chance
I don't know how i can do this. It is a bit hard for me.
in L2PcInstance i do this
dropQuestItems *= calcPremiumDropMultipliers(itemId);
It is good?
I do this in L2PcInstance
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