Ive been doing alot of reading into hair follicle tests recently and im quite surprised they allowed you to do the test with only 1/2 inch of hair, they usually require 1 1/2 (which goes back 90 days roughly). you should be fine mate, although its very hard to guarantee anything. Ive seen no basis to believe that any hair cleansing products work at all, the THC giveaway chemicle is in the hair follicle itself (actually part of the dead cells) so i cant really believe that there is any product out there which makes any difference what so ever.
All the best mate, I think youll be fine.
mrmarijuanaman