I don't think anyone has mentioned this yet, but the Radian RXT manual specifies that the child's head cannot be 1.5" from the top of the shell, not the usual 1".
http://us.diono.com/docs/manuals/Diono_US_RadianRXT_Manual_US_Eng_Web.pdf
Rear-facing child is too tall for
restraint if top of head is less
than 1.5” from top of restraint or
if child is more than 44” tall
(112 cm).
http://us.diono.com/docs/manuals/Diono_US_RadianRXT_Manual_US_Eng_Web.pdf