Double loot instances just double the rolls made against the loot table selection. Like if a mob has a 100% chance to drop something like a armor pattern, the game makes 2 rolls and drops 2 armor patterns from that mobs 100% drop loot table. It then makes twice the rolls for items like rainbow crystals, essences,plat boxes etc. So if the mob say has 4 possible corpse loot slots in a regular instance, and 1 slot is your 100% drop token/armor pattern/key/master essence etc. You now will get 2 100% drops. The 3 remaining slots in a normal non double loot, will now become 6 loot rolls against whatever the mobs remaining loot tables are.
Example
loot table 1 100% chance to drop item(s) 1a 1b 1c
loot table 2 50% chance to drop a item of type 2a 2b 2c
loot table 3 25% chance to drop a item of type 3a 3b 3c
loot table 4 10% chance to drop a item of type 4a 4b
loot table 5 .000001% to drop a item of type gss or hammer piece
loot table 6 15% chance to drop nothing on loot roll
Now you have 6 chances to random against landing on loot table 2-6. Inside each of these loot tables if your roll hits the table, you now make a second random roll to determine which of the items inside the table you receive, with each item inside of the table having its own %.
Example
Roll 1 lands on loot table 2. Now a second roll is made to determine.
2a 50% resist stone
2b 25% plat bag
2c 25% plat box
I have no idea what all that fancy letter math is you wrote but this is how everquest draws from its loot tables.
