Well, I thought there was a total of 7x random-direction nodes that I need to pass. 0,5 times itself 7 times is approx. 0,0078. But I actually just noticed that the first RNG node (bottom left) is irrelevant as both paths lead to the first mob (hadn't noticed), so I guess it's a total of 6, in other words 0,5*0,5*0,5*0,5*0,5*0,5 = 0,0156 = 1,6%. We're doing the same math, I'm just writing it in a less than "intelligent" manner, but I'm sure you understand. But, there aren't 4 coin flips, there are 6 coin flips in the level.
Hm, wait, I might be thinking about this all wrong.. Basically I multiply by 0,5 for each unit that passes a 50/50 node. There's two paths at the top of the map, that's 0,5*0,5. Another two paths at the very bottom, 0,5*0,5*0,5*0,5. And finally I need two units to actually get down TO the fork at the very bottom, so that's another two: 0,5*0,5*0,5*0,5*0,5*0,5 = 0,0156 = 1,6%. If I'm way off, I'm sorry! ;;
By the way, thanks everyone for your input. I now know how to proceed with the missions, and I'll be sure to never skip the 80 stamina secret garden!