I just brought a Manduca so that I could back carry my 15month old (I am tired of her clinging to my legs!

)
I read the instructions and it says that you can only back carry after 8kgs. She's a little dot - only 7kgs.
I am tempted to ignore this and use it anyway - she's very strong - hardly a floppy newborn! I only want to do the back carry.
Anyone else back carried before 8kgs? Is there a safety issue that I am missing?
Thanks