Converting KCAS to KTAS for supersonic airspeeds

[delta2017]

I have been using this Atmospheric Properties Calculator to convert KCAS to KTAS:

http://www.aerospaceweb.org/design/scripts/atmosphere/

I asked someone if this calculator was accurate. He said for subsonic airspeeds, the calculator should be accurate. But he said it probably is not for supersonic airspeeds. He discussed why it is very problematic to make an accurate KCAS to KTAS conversion for supersonic airspeeds.

Does this particular calculator accurately convert KCAS to KTAS, even at transonic and supersonic airspeeds?

Thanks,

Delta