My DD is 18 months and she is in 95th percentile. She is using SureRide as a spared one on my car. According to manual, the lower three slots are for RF. The top three slots for FF only. Right now she is well pass the last slot of RF. Just half of an inch lower from the bottom slot of FF. Can I use the bottom slot of the top three for RF? She seems uncomfortable with highest slot of RF. Not sure if I can keep her on same slot till 4 YR old.