I have a small area against a wall where it dips down about 1/8" towards the wall about 5" from the wall. I filled it with 1/8" underlayment and glued it down using construction adhesive and sanded it as smooth as i could get it with a belt sander. Where this dip is, is where my starting row will be that i will be gluing and face nailing so i want it to be as level as possible.

So... there is still a bit of unevenness around the area (about 4 foot long) where i went a little to far with the belt sander and where the underlayment i glued down meets the 3/4" plywood. So i had an idea of using HD construction adhesive as filler. just squirt it on and use a drywall knife to smooth it out like filling low spots on a car with bondo.. Is this a good idea?

Sounds a bit messy, since you are nailing the flooring down why don't you use felt paper and just cut out pieces and build up the floor that way.