Bobdazzle,
The correction is a proportional one and not a single fixed number. The amount of the correction reduces as one descends towards the airfield.
EXAMPLE:
Airfield elevation sea level
OAT -25C
deviation from ISA= -40
Cold temperature correction = 4 x 40 per 1000ft above the airfield
(i.e. 160ft per 1000ft)
If you had a 'STAR altitude' of 5000ft, the correction would be 160 x 5 = 800ft.
You would fly at 5800ft (having advised ATC first, of course)
At 'descent point' of say 2000ft the correction would be 160 x 2 = 320ft.
You would fly at 2300ft
At the FAF altitude of say 1200ft the correction would only be 160 x 1.2 = 192ft so you would fly at 1400ft
At the MDA of say 300ft the correction would be 160 x 0.3 = 48ft so you would fly at 350ft.
I may have mis-understood you but you seemed to imply that you had one correction figure that you applied to all phases of the arrival and approach.
You can see from the above that the correction value reduces as one gets closer to the ground. It is most important at MSA and hardly relevant at most MDAs or DAs.
If you applied a single large correction for all phases, it would explain why you were high at the final approach fix. The fact that the correction is actually rather small at the FAF would explain why you did not feel that you were too low when you did it the 'old fashioned way'.
Hope this helps.