i use the outter head of the tri (easiest one to get to) and usually just hit just slightly to hte inside of the peak of the bicep (like 1/4" off center at most)

i love bis and tris. I have no pain at all in the tris and only some slight injection pain at bis. Both recover fast and i don't even know i've injected the day after. i don't use anymore than 1.5cc in each muscle though.