I have a casserole recipe that I generally cook in the oven.  I'd like to try it in my slow cooker for several reasons:  convenience, timing, opening up the oven for another dish.  Is there a general rule of thumb to convert the directions for the oven to an equivalent for a slow cooker? 
Roughly speaking, the low setting on a crock pot is 200 degrees Farenheit, and the high setting is 300 degrees.   Crock pot time vs oven time:   4-6 hrs on low = 15-30 min oven 6-8 hrs on low = 35-45 min oven 8-18 hrs on low = 1-3 hrs in oven   In addition to the liquid notes above, you may want to make these changes as well:   reduce the amount of whole/leaf herbs by half add ground spices during the last 30 min of cook time you may prefer to brown meat before cooking in the crock pot; it's a flavor & fat issue add rice or noodles in the last two hours of low cook time  