Floor Jacks / steel support columns

My 70 year old house has some slanting floors on the second story. The two original wooden basement support columns were replaced by a previous owner with steel adjustable pillars (jacks). I believe new concrete footings were poured before placing the new steel posts. Questions: 1) How deep should the concrete footings have been poured (Chicago area soil/clay)? 2) Can I slowly (over the course of many months) start rotating the threaded extension rods in the steel jacks to try to reduce the previous settlement? If so, how much can I extend the posts each time and how long should I wait between adjustments?

Any info would be appreciated.