It's 12.5% per unit sent that way to be sent the correct way past the 3 gates 1/2 x 1/2 x 1/2 = 1/8 = 12.5, but you can garuantee 2 units get sent that way since you need at least one unit for the other route and considering, but considering the even numbers get sent right instead of left you only need to start 3 units at the guaranteed gate to send the 2 units upwards for the increased chance to fight the top boss.
For those interested in powers needed to win my team power was
1) 61k (1 5* and 2 4*)
2) 23k
3) 20k
4) 20k
H) 40k
All teams were about 400 speed. (450 for main and under 420 for the other 3, Helper was around 400 too)