FYI

  dynamic_pressure = 0.5 * ref_density * equivalent_airspeed^2

Its not a function of indicated airspeed. Indicated airspeed needs to be corrected for the compressibility of air to obtain the equivalent airspeed. At low airspeeds/altitudes, the indicated airspeed and equivalent airspeed are very close, so your equation would hold up just fine.

As a result at higher altitudes and faster speeds, the red line on an indicated airspeed gauge would be variable to account for the effects of compressibility.