Sounds like Rhodri is doing really well, all quantity's are guide lines only as each dog is different, it depends on how much he is burning off during the day etc,.
You can always up or down his food if you need to, I found I was constantly worrying about Bea's weight, was she fat,did she have a waist, was she hungry ?
I ended up taking a photo of her from above so that I could see how she looked, it's so hard when they are pups as all I could see with my girl was fur
They grow so quickly , sometimes you are just upping the quantity every few weeks until one day you realise that you don't need to anymore.