How should feedback be given for "silly mistakes" on an exam I am a TA currently grading a set of 80 midterms for an engineering math course for fourth-year engineer undergrads. The test had four questions, each question had around four parts. 

Of course, typically a student can make a wide variety of different types of errors on the exam. But what I have noticed in this exam is that some of these errors are not necessarily conceptual, yet we are basically taking all the points away for these "minor errors" (per instructor's grading scheme).

For instance,


For a question with several parts. A student copied one of his own
(correct) answers from the previous part incorrectly.
A student wrote down all the steps correctly, until the last step
where he had to evaluate the expression at a number and the mistake
was forgetting a negative sign.
After a long derivation, a student substitute formula incorrectly (forgot a square root in the formula, ahem, Gaussian). Everything else completely correct. 


There was a litter of other minor errors. Now, these students are in their final year of engineering school, so I think I can give them the benefit of the doubt that they wouldn't make silly mistakes such as writing "0 - 1 = 1" or copy things down wrong in a non-time-constrained setting, which is the situation they will most likely face when they graduate and work in industry. 

Is there a way to provide feedback for a student if he or she makes silly mistakes on the exam? I do not think that giving them a zero for writing a minus sign incorrectly is a good way of either providing feedback or preventing the same mistake from happening again in their (long) lives.
First, notice that students will typically be able to tell which of their mistakes were silly and which more significant.  So for silly mistakes you don't need to write feedback other than to circle where they went wrong, and indicate the number of points taken off (or granted).

For a more serious mistake you can write more detailed feedback.  Sometimes feedback here isn't necessary, either, other than to write, "see solution."

Second, you can figure out what types of errors and what frequency of errors you want to bring the grade down from A to B, from A to C, etc., and decide how many points to remove based on that.

More importantly: what's a well written exam like?  Example: I'm asking them to solve this eigenvalue-eigenvector problem.  If the student has a vague idea what is being sought, I want him to get one point.  If he sets things up well but doesn't know what to do next, he gets two points.  If he follows through well but for whatever reason didn't quite get to the perfectly correct punch line, he'll get 3 points.  Perfect, complete answer: 4 points. (This is just an example of the point distribution.  You might end up with a different scoring structure.) In short, the scoring should be integral to the exam design from the beginning.

Most importantly: as a TA, you should be getting special guidance from the professor for grading a midterm exam.