You know the increased chances don't go into effect until you pull a champ of that rarity first, right?
So if you you have a 15x chance to pull the epic you picked, it won't go to 20x chance until you pull a different epic first, no matter how many shards you pull total
Same for legos, it won't go from 15 to 20 until you pull a different lego than the one you picked, and it won't go to 25x until you pull a second lego that isn't the one you picked.
According to Ayumi, there are 226 non-void epics in the game right now. A handful of them can't be pulled from shards, so call it 220.
So if you get an epic from an ancient shard, your odds of pulling, for example, Fenax would normally be 1 in 220, or 0.45%.
During a 15x event, the odds become 15 out of 234, or 6.4%.
If you pull a different epic, then your odds of getting Fenax go to 20x, which is to say 20 out of 239, or 8.4%