small square diag is 2*sqrt(2) (Pythagoras)
large square diag is 4*sqrt(2)
distance from small square corner to lower left large square corner is R+x
x = sqrt(2)*R (Pythagoras)
-> 4*sqrt(2) = 2*sqrt(2) + R + sqrt(2)*R
solve by R: R= 2*sqrt(2) / (1 + sqrt(2)) = 1.17