it is not the altimeter only
1. some altimeter show a wrong reading when you fly slow, or with flaps. There should be an Altimeter Correction Table in the POH.
For the King Air 200 the Altimeters are 20ft wrong at 110 KIAS and 0 Flaps at sealevel, they are 30ft off with Flaps 40% at 100 KIAS at SL.
Another point is the GS Indicator, it is allowed to deviate 0,2 degree as far as I know. For the OM is usualy 5 NM from the GS Antenna, 0,2 degree gives a Mistake of about 100ft.
sin 0,2 * 5 NM
100 ft Error sounds horrible, but thats the error at 5 NM only, it will become better, the closer you get to the antenna..