50% chance comes from my own experience... providing You are using compasses in most efficient way. Just remember that if You have mounts on 4 cards out of 8 it doesn't meant You have 50% chances of rolling one. Apparently it looks like each card has different chance of success roll and the rarest one have lowest chances so a set of 8 cards are only showing what You might win during draw beside stuff majority of the ppl are calling junk.
Let me give an example. After using one prophetic card preview shows: legendary mount, legendary box of cc, regular box of cc, clothes, helmet, common box of cc, carnival mask, mount food. Now what it means that those items have been added to roll list, each with different chances of success (don't think that You are actually drawing a card from that set) which explains why when You get stroke of luck full with legendary prizes You are still getting junk from the roll most of the time. Therefore using more than one compass per card is a waste...