The instructions for adjusting the blade depth are in the learning center
http://learn.silverbulletcutters.com/ac ... ck-holder/Using this as a resource is so important as it shows how to do it so much better than any of us can explain!! Yes you are right about the importance of the setting and the force - it's the difference between a good cut and a disastrous one.
There should not be a lot of play in your mat already. Are you rotating it so the pinch wheels are not in the same place all of the time and storing it flat. I have been using the same mat for months and mine is still really flat. That being said, no dont push the setting tool down - just rest it on the paper on the mat, and let the blade just touch the setting tool. What I do is let it fall all the way down (to where it is hitting the setting tool too hard - so if I pulled it out it would scratch the tool ) and then I pull the blade holder up just a little so I can easily move the tool out of the way and set the clamp on the blade holder.