I think you have to unwrap the cube then it is more simple. It’s then sqrt(1^2 + 2^2 )=sqrt(5)=2.2360679775

I’m sure there’s a more technical explanation, but my assumption is two lines forming 45 degree angles at the corner of the points so ((1^2+0.52)^0.5)*2= 5^0.5 or square root of 5 or roughly 2.236

Sorry for the formatting. I’m on mobile and missing some symbols that would make that easier to read