Basically there is toughness that estimates the overall ammount of damage you can take before dying. The problem is that it includes your HP in the total toughness calculation. The damage resistance you are talking about is called mitigation and it can be calculated using one of the various tools out there. I believe Bagstone wrote this http://d3resource.com/mitigation/ for this specific reason.
Original thread: http://www.diablofans.com/forums/site-related/diablo-tools/85936-mitigation-calculator