I am going to do the Exmoor Beast extract kit from Brew Uk as my next (second) extract brew. The first was Way To Amarillo which suggested 3.5g of sugar for a 21 ltr batch. I batch primed that with slightly higher than 3.5 / l: I used 85g of brown sugar in a priming solution in a bottling bucket: and the bottles all turned out to be completely flat.

I don't want this to happen again, so can anyone suggest an appropriate priming rate for the Exmoor Beast please- the kit is supposed to make 15L.
Much appreciated!