Dont worry about trying to get the red dot to shadow the front sight, you need to independently adjust the red dot to your POA and focus on that. If the red dot happens to shadow the front sight then great, but dont just put it there. I run an Eotech on my AR and i have it adjusted spot on but it doesnt quite shadow my front sight. Are you able to co-witness with the reflex?
By the way how are you liking the Pentax reflex? I had a Pentax lightseeker 3x9x50 scope on my winchester M70,and it was an amazing scope.