Elementary CAS vs TAS q

[ExcessData]

Hi all

Fairly simplistic q, I know, but was talking to a mate the other day about the well-known CAS vs TAS relationship, and we were trying to work out what the TAS will do, relative a constant CAS, as you descend 'below mean sea level' (ie. a hypothetical case).

Any input greatly appreciated.

Cheers, ED

[Edited for syntax]