FNW,
Have you actually tried this or read the manual closely?
You cannot change the PFD sub scale from the CPDS Config page.
Software change in the FCDS only -
If the configuration file needs to be written it is expensive. That depends on your avionic install config and whether it has previously been coded with the scale type required.
Easiest fix - replace the 2" STBY ALT with a dual sub-scale one.