By using a float param as a counter in the loot table, you can garauntee that a particular item is dropped at least one time out of 10 times:
Decrement the counter
Test the counter
3a. If counter is less than 0, go straight to garaunteed treasure drop
3b. Otherwise, go to random choice (random choice has a chance of also dropping the treasure)
If you drop garaunteed treasure, reset the drop time counter back to the max
By using a float param as a counter in the loot table, you can garauntee that a particular item is dropped at least one time out of 10 times: