Wilfred.. you are getting there .. as I suggested earlier and vapour trail reiterated .. the problem relates to how you simplify things .. CL depends on a number of things .. including Re (Reynolds Number).. at altitude CLmax is reduced ... solving for stall speed then gives an increased value for Vs .. don't get too hung up on EAS .. it is basically something we engineer chappies need to make the maths a little easier to process ... basically EAS is a "standardised" TAS .. all the other "speeds" are the system's way of getting around the fact that the ASI's calibration lets us down anywhere other than at standard sea level