I don't have the reference here but I remember the altimeter error of 60' be extended upto 75' (or something close) provided;
-you have 2 altimeters
-you check it again at the next possible chance
If I remembered correctly, then is it safe to say that 75' would be the maximum "discrepency" your allowed. As the altimeter would be U/S otherwise..
Besides if your off-slope, then you wouldn't be expecting the correct check height in the first place, therefore it's not an unexplained discrepancy
(just fix it)