+/- 50 to 100 ft is fine. Accuracy will come with practice and then will fall off while you focus on the next learning task. Then it will come back while doing whatever the 'new' task is then it will fall off at the following new task etc etc...
If you find you're consistently 50' or 100' or whatever too high or low ie the same error then start your control input slightly sooner (if always high) or later (if always low).
You might also find that if you are consistantly climbing above the target altitude then a **ROUGH** ie not accurate, use of trim while accelerating may help. Be aware this is NOT the same goal as normal trimming (hands free flight), it's just to reduce some of the control pressure you're holding while waiting for the speed to stabilise.
Some exercises with your instructor increasing/decreasing power &/or flap setting &/or trim settings while you concentrate on holding specific pitch attitudes & headings could also help.
Definately don't stare at the instruments. It really *IS* an outside reference manouevre. Altimeter is just to tell whether your 'outside' job is as accurate as it could be.
[ 17 July 2001: Message edited by: Tinstaafl ]