I just got a new disc wheel (11 speed) for my sram Redd 10 speed TT bike, so it needs a spacer to get it to work with 10 speed. It came with two spacers one is thicker than the other.

One has a 2 something on it. I assume this is 2 MM. The other one is thinner and has no thickness marking on it. What does the thickness of the spacer need to be to move a 10 speed cassette to the right location to work with a 11 speed compatible wheel?