I was reading up and on the NeuRA site has an article about the shoulder height markers and it says

“Shoulder height labels make choosing the right seat as simple as choosing the right shoe size,” says Dr Brown. “If they’re over the line, you move them into the next size and if they’re under, you stay in that same seat until next time.”
So those shoulder markers have now come in, but nowhere does it say what the change to booster means - on the Neura site it says

6 months – at least 4
years
Until shoulders >2.5cm
above top harness slot

But the top harness slots are way above the change to booster mode. Now I don't know if this is because of the Protecta type harness which can fit into slots on this booster - is that why they have those slots - why have them if you are meant to have changed to a booster already.

The site also says
“Parents have to wade through complex instructions involving weight, height and age to find the right seat for their child,” says Dr Brown. “It’s no wonder they sometimes get it wrong.”
I am not convinced using the height markers only has really helped, I do think they make it easier to see when a child is ready for or has outgrown their seat but in terms of choosing one the elimination of the weight I think has made more confusing for children who are taller/heavier than the average.