I've always based it on weight and shoulder height. DD2 is outgrowing her 5-point harness toddler seat based on her shoulders (can't move the straps any further up) so I'll be getting her a new seat in the near future, even though her weight is still OK. Just as well, as DD3 is getting a bit podgy for the infant carrier.....
which car seat have you got, because I had a britax renaissance that was bigger for tall children, there is also a chicco one that again has a very tall back. They both fitted longer than the elvova 123 using the 5 point harness.
I had tall dc, it's a nightmare!
I've actually got a speacial needs one for dd4 now to keep her in a 5 point harness longer.
dd is nearly 4 104cm (if not more) and she still fits in the middle shoulder slots!!!! I ended up getting it as I can using the tethering straps to make the car seat secure in my lap belt in my acient espace much cheaper than buying a new car.