i think it follows from definition X(t) - X(0) ~ N(0,t).. X(t/c^2) - X(0) ~ N(0,t/c^2), from here we can compute dP(cX(t/c^2) -cX(0) <=z)/dz (even this is not needed.. the specific corollary for normal variates can be used if eps~N(mu, sig^2) then k*eps~N(k*mu, (k*sig)^2) ))