Worth mentioning that most pilots don't actually care about any of the above, with the main objective being to keep out of controlled airspace
So if flying below CAS whose base is defined as a
flight level, you fly on
1013.
If flying below CAS whose base is defined as
altitude, you fly on the
actual QNH, but that gets a little tricky because the regional pressure setting is no good for staying below CAS. The "proper way" to stay below CAS whose base is defined as an altitude is to obtain the QNH from the ATC unit responsible for that piece of CAS (which you can do by calling it up, or by dialling up its ATIS).