Re: smaller babies in convertibles
Just to double-check - you need to use the "rear facing boot" when using the Radian rearfacing. It is a plastic/metal piece that fits on to the foot end of the car seat in order to make the angle more reclined. I assume that you are using it already, but best to be sure!
The radian can be reclined as far as 45* from vertical when rear facing. If the angle in your car is naturally more upright than that (while using the rear facing boot), you can use the top tether (which is approved for use when rear facing, unusual for convertible seats, but okay with Radians) to pull the seat to a more reclined angle. (Note for readers with Britax convertibles: Britax also allows top tethering when rear facing, however they do NOT allow you to adjust the recline angle using the top tether.)