IMO, if it says 35 lbs, then a 35 lb child can RF safely. I know they have to engineer it with a margin of error -- just no way of knowing for certain at what weight exactly it will fail so we have to go with published limits.
I would let a child ride it in until the first time they weight in at the limit (with a full tummy & fully clothed to include shoes). Assuming you have a scale at home or otherwise are able to weight often, at least 1x a week. If you have to really on Dr's visits or WIC appointments or some such for a weight and can't get them weighed more often than every few weeks, then switch at a lb or two below. If you go months between weigh-ins, then better to switch earlier.
ETA - in talking to an avg parent (ie not a carseat addict) though I'd recommend switching a bit sooner; ie a 33 or 34 lb kid in a seat that RFs to 35 at a check, I would probably recommend to FF, unless the child is very young (ie, 1 yr), to make sure they did not go over the weight limits before turning.