A 40 is pretty fine almost like what use for clipping before a surgery....I think.
Start with a couse blade first like the 10. If you want to buy a blade start coarser than that and see how it works. If you get too fine of a blade, depending on the speed of your clippers and the coarseness of the coat and thickness of the undercoat, it will be tough going.
You would think someone on here is a dog groomer could answer this! LOL
I am speculating!