How many marks should I take off for a student using an incorrect value?
Marks are a measurement of how well the student knows the subject. So it depends on how much of a lack of knowledge that mistake represents:

in most cases, if all steps are correct and only a number was copied wrongly, then obviously the knowledge of the subject is very good, and most of the question was done correctly, so &quot;almost full marks&quot; are deserved

if the mistake shows lack of knowledge (for example, if that &quot;-64&quot; is a value of power, which can only be positive, or if it leads to an obviously unphysical result that the student should have noticed), then one might remove more marks.

a tricky case is if the mistake leads to the problem being much easier to solve (e. g. &quot;Write the last 10 digits of 2^(64+x)&quot;), and it is correctly solved. In this case, it would be unfair to give full marks, as the student did not show much knowledge of the subject. Unfortunately, this should probably result in no marks.


It seems like your case falls in the first category, so I would see no reason to give anything less than &quot;the highest mark that is not a full mark&quot;.