90% oxygen would be badly hazardous, remember Apollo-1. Use 90% air instead.
How much helium... One half-tone is a 5.9% frequency change (that is, 1.05912=2 since Bach), so maybe 20% pitch suffices - no accurate estimate. If you just neglect helium's density (not the biggest error here) and effect on compressibility (monoatomic vs diatomic), 20% velocity change needs a 40% lighter gas, or 40% He and 60% air. A nice but high-tech mix would be 20% O2, 40% He, rest N2.
Zero oxygen in the breathed gas isn't immediately noxious. Pilots make such trials to be aware of the effects: euphoria after several breathes, then fainting. The apparatus and your posture should take the helium source away if you faint, so hold the source with your hand instead of a strap, sit on a bed, have an other person nearby - I mean, a friend.
Double-check if helium may dissolve in the blood. I believe O2 + He is used for very deep diving, to avoid the soluble N2.