What do I need to show to get an adjustment/modification of child support, especially a credit or deviation?

Court ordered child support based on imputed income (i was a recent graduate and only had found part time work at the time), I now have full time work (which is less than what was imputed). More importantly, I have my son 45 percent of the time, pay all expenses while he is with me (food, clothing, entertaininment, transportation, medicine, etc), split all child care costs, school costs, and activities, and am paying his Dad (who has him one extra night a week), over $600 a month in child support. I understand the general rules/procedure for getting it changed, but don't know what I need to show to get a credit/deviation from the general order for all the expenses I incure while my son is with me.