First off, congrats on the rescue and kudos for recognizing Beneful bites

. I would say you need to start by googling raw feeding and read, there is tons of info online and a lot of breeders who do raw have sample menus. Typically you would feed 3-5% of their adult body weight. I believe my Rock will be about 75 lbs. when mature so that would be 2.25 lbs/day. He gets 1.12 lbs. per meal (am and pm). They would get 80% meat, 10% bone and 10% organs.
Most people start out with meaty chicken as they are nice and soft or else chicken quarters for a couple weeks. If all is going well, slowly add other protiens one at a time. You can then slowly introduce orgarns (liver, heart, kidney, etc.) Tripe is also good as are eggs, salmon, sardines, mackerel and so on.
Their stools should be firm but not chalk white as that is an indication of too much bone. My crew typically gets ribs on wed evening and a back/shoulder bone on sunday night and the rest of the week it is just meat, tripe, organs. Once in a while I will do up a veggie mush (pureed veggies) and give some of that .
Good luck, do a lot of reading and make sure you are comfortable before starting. You may also want to find some good sources so you are not at the grocery store every other day.