I'm astonished that there would be any circumstances where the addition of flap would allow you to glide further. I have never flown an aircraft where any additional flap will increase the gliding range (unless you have a very strong tailwind and it allows you to stay in the air slightly longer because of the reduced sink rate). I have done very little flying in cessna trainers but have over 5000 hours in a mixture of gliders, light singles and twins, light single pilot jets, and various multi crew jets. Pretty sure the best glide range is clean in everything and the addition of any flap at all will increase drag and decrease range.
Heres my advice chap - if you are at a sensible glide speed and the reference point is moving up in the windshield you are undershooting. Pick a new field as you aint making your chosen one..
nota bene.. don't turn a training event into a reality. Trust the feeling in the pit of your stomach. You knew this was going wrong a long time before you threw it away. Learn to trust that feeling because it will save your life.