Going to BBB, what harnessed boosters to try with DD1?

[Joolsplus2]

Is the limit the weight of the child, or the combined weight of the seat and the child? (If just the child, we'll be able to LATCH DD1's seat forever, ha!)

That's the weight of the child. The math was done so the seat plus that weight is 65lbs (the vehicle's max weight).