If you visualise the CL curve then CLmax (or Vs) will be at
critical alpha or the very peak of the graph. 1.3Vs is faster than Vs so it
must be at a smaller CL. To determine the percentage you need to rearrange the
lift formula.
L = CL x Vsquared (all other factors remaining
constant)
So CL = L/Vsquared
We dont know the lift but at Vs we can assume unity
so CL = 1/(1.3)squared and then x100 to get a percentage =59%