Error of Observation
In seeking with the knife-edge the center of curvature of any single zone, which is done through visual interpretation of the shadows cast on the mirror, you cannot, with certainty, repeatedly place the knife-edge within 0.01" of the true position.
A skilled observer can probably halve this error, but with the average amateur, who may make one or at most two mirrors, the error might easily be doubled. Also, assuming that there is no error in the scale, there is nevertheless a possibility of error of 0.01" in reading it.
Of course, where the knife-edge movement is controlled by mechanical means, this latter error should not exist. But as two zones are involved in measuring the mirror's correction, the worker should be prepared to allow for a total error of 0.02".
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