Your chin should be tucked.  This stretches the suboccipital muscles.  These are the muscles at the base of your skull.  This is the functional centerpiece to all the muscles that run parallel to the spinal column.  It is also functional centerpiece to the hamstrings, calves, and bottom of the feet.  All these regions respond to the conditions at the base of the skull

Your ear hole and your shoulders should line up in the same plane.  This is neutral position.  For every inch your head is forward from neutral position, it is like adding 10 pounds on your head.  

Phone

Please don't hold your phone with your shoulder when you talk.  We can not tell you how many office workers' skulls we have to adjust on a weekly basis, and treat the related musculature

To correct slouching

The best way we know how to correct this to get a non-elastic back up chair from nadachair.com

This actually keeps all your spinal curves in the correct position.