I remember where I see the info about psychic levels in the Imperium :[link]
As Rho and Pi are normal humans, and Iota the minimum level where a psyker is esteemed worth to sanctioning, I figured that Boone must be between Omicron and Kappa.
And one last question : if Boone has lived in hiding, haw she has been sanctioned? She couldn't have been in the Scholastia Psykana for training and sanctioning, as they would imediatly found she's too weak for sanctioning? Or it exist some king of "clandestine sanctioning"?