Generally speaking from my experience as a 285+ bencher at 180, you should touch to your upper abs while tucking your elbows so that there is a straight line between your elbow and the bar, and your elbow is vertical at the bottom.
As for pressing off of the chest (I assume you mean sternum/nipple area?), if you want a shitty bench press, an inflamed bicep tendon, and many pec tears then you can press off of there with a wide grip. As for how hard it is, it depends on what is muscles are strongest, and your proportions. I have very long arms, and very strong triceps and deltoids so my wide grip bench (pinkie on the power rings) is less than my competition grip because I can't use my triceps and deltoids as much.