Air slash normaly causes flinching 30% of the time. With Serene Grace, that gets multiplied by 2 for a total of flinching 60% of the time, or moving 40% of the time. The evasion is raised 10% due to the bright powder, givig you a 90% chance of hitting. Head smash has a 80% accuracy to begin with; if you don't flinch. To combine these threefactors, we do some multiplying:
40/100 not flinching x 80/100 accuracy x 90/100 evasion = 288000/1000000, or a 28.8% chance of hitting. If I did the math right.