This is *almost* correct. Head Smash is 80% accurate and BrightPowder makes you 90% accurate on normal moves. Therefore it's 0.8*0.9 = 0.72 = 72%. Coupled with Air Slash that's 0.4*0.72 = 0.288 = 28.8%, closer to 29%.
EDIT: clever Fritjof, you beat me to it!