Consider a rare, recessive, deleterious allele a
    f(a) = q << 1   &   f(A) = p ~ 1
    f(Aa) =  µ  mutation rate (# new mutant alleles / gamete / generation)

The value of q is an equilibrium between loss of a due to selection
                                                                   & replacement of a by new mutation

change in f(a) due to selection: q_s= -spq² / (1 - sq²)
change in f(a) due to mutation: q_µ= µp

Then q_s  +  q_µ=  µp -  spq² / (1 - sq²)
                              µp  -  spq²     [since  (1 - sq²)    1   if   q<<p ]
                            =  p (µ - sq²)

At equilibrium () : q = 0 =  p (µ - s²)
                                                  µ - s²     [since p  1 ]
                                      s² =µ
  So             =  (µ / s)^(1/2)