I have rows for all the possible cards, the total number of packs and the mythic drop rate. I just adjusted the number of packs opened until the the total number of Mythic + Mythic Wild cards was = 80 (4 copies of the 20 total Mythics).
The simplest formula is:
Total Mythics = (Packs/7.4) + (Packs/30)
7.4 is the drop rate (you get 1 Mythic on average in every 7.4 packs) and Mythic WCs in packs replace only Mythics so you can count them as the same here if you're willing to spend them.
Packs/30 is for the Mythic Wild Card you get from the Wild Card wheel for every 30 packs you open.