Mixing ammo, particularly ball and civilian hollowpoint in a AR mag, doesnt really make much sense. Both types are more than capable of dropping any person with a solid torso hit.
The idea of mixing ammo types in a mag or a belt comes from the military. Of course what isnt noted are the shortages of various types of specialty ammo that forced armorers to resort to this. There was never enough AP or API on hand so it was mixed with ball to streatch the supply.
Tracer appears 1in5 in MG belts because of the incresed cost of the tracer bullet, not because there is anything magic to this ratio. Lots of shooters dont like tracer at all because it is a two way street, they mix and match belts and mags to take them [i]out[/i].
For an AR-15, if you want to change ammo, then change mags, dont bother with mixing, cause its unlikely (unless you handloaded both types to match) that they will strike close to one another. Any 55+gr load from a AR15 is more than effective on a person, ball or HP, provided you do your part with your aim. Given that, why go about doing something that could make consistant aiming more difficult?