If you are a pilot, then apply common sense. Simple question - Is a chart corrected for below ISA temperatures and below standard pressures, or does is state elevation as being above MSL? If you believe the former then do nothing. If not, make some corrections. We use 40 feet per degree below ISA plus 30 feet per HPa below standard. We typically over-correct because we correct for the entire column of air between or cruising level and MSL and not just the layer between us and the ground.
CS - You missed the bit where do you take away the square root of the number you first thought of.