I'm with Bill. When I shot FITA target my anchor was the top of my index finger, tight upto the lower line of my jawbone. I found a 'platform' type tab really, really helped to get a good solid reference tomy jaw. I also had kisser on the string as a second reference.
Just be aware, anchoring under your jawbone can reduce, rather than extend, your drawlength. Mine's stretched by a 1/2"