The best place is not always the best practice when choosing where to place the seat. The middle back seat is the safest if the seat can fit there and there are no other problems. The next safest seat is the seat behind the driver. However, there is only a slight difference in the safety numbers for being behind the driver compared to the seat behind the front seat passenger. The backseat is the safest place to seat children.
Most crashes are front end crashes.
After 20-22 lbs the child has grown too large for this style seat. You must choose a different style seat. Keep harness straps snug and fasten the harness clip at armpit level. There should be no more than one finger width at the shoulders or chest. Straps should be at or below the shoulder level.