What would a good written policy for scaling exam grades look like? I'm not in the habit of scaling exams, although I'm aware that the practice is widespread and highly recommended in some circles (such as grading on a curve). To my recollection, none of my college instructors scaled exams or wrote any policy about such on their syllabi. I'm wondering if one did commit to an official policy of scaling exams, what would a reasonable written policy look like?

The goal here would (hypothetically) be similar to Krantz in How to Teach Mathematics (Sec. 2.10): "My main goal in formulating my grading policies is to make the greatest number of students feel that they have been treated fairly (and, not incidentally, to reduce student complaints)." Secondarily (and again hypothetically), to defend against possible future administrative complaints that the instructor is not matching some recommended grade proportions (same section by Krantz). 

Please assume that the individual exams are adequately fair assessments themselves (not broken, vague, or overly-hard questions, cover same topics covered in class and homeworks, sufficient time permitted, significant advance time taken assessing questions and grading rubric, etc.). 

Background: I'm at an open-admissions community college where high failure rates are historically common, often half or more of many math or computing courses. Among the things I'm worried about if I started doing that are: Would there be any lower bound to what might wind up being passing work? 

For example, I had a colleague at a different university (top-20 in U.S.) who got in trouble a number of years ago in that, when pressed by students on his scaling policy, said that if every single student got a zero on a exam, then the scaling process would turn all of them into 100% marks. Surprisingly, the students successfully organized a total boycott of the final exam, and my friend followed through and gave 100% marks to everyone as per his word. (This turned out to be quite embarrassing for him.) 

So I'm wondering what kind of formal, mechanical policy for scaling would prevent no-lower-bound situations similar to this one?
In general scaling is a mistake as it introduces uncertainty into the student's calculations. But some adjustments are proper and you can publish statements about them. 

First, you can agree that no one will "miss" the next grade by a small percentage. This is better done overall, than on a per exam/paper basis. But if it takes 90% (overall) to get an "A", then, at the end of the day give the A to someone with 89. This merely recognizes that your grading scheme isn't perfect and may disadvantage people in small ways occasionally. 

Second, agree that your grading rubric is clear that you are allowed to adjust upward when you think it justified. Mine would say something like, "If you get 80% you will get at least a B". This is just a reinforcement of the first point.

Third, permit people to do work over if they have fallen short. Regrade it for "most of" the lost points. My policy was 90%. If you lost 30 points on an assignment you could get 27 points (max) back if you re did the assignment properly. My policy was generous, permitting several attempts. The repetition was good for the students and worked to assure learning. 

Fourth, at the end of a course, look at how the students did overall and compare it mentally with what you think they really learned in the course. If you think the learning was actually better than the distribution shows, bump it a bit. This will push a few students up to the next partial grade, say from B to B+. 

Finally, avoid marking using only a few high-risk exams, but spread the marks over a large number of tasks. All or nothing final exams eventually leads to trouble, even in less extreme cases than the one you mentioned. This leads to a practice of continuous study and learning rather than "cramming" for the big one. Cramming results in more memorization (short term learning) and less deep learning. 

My students all knew everything about the above policies. I seldom got complaints about grading. I could be as demanding a professor as I felt necessary (students viewed me as very demanding). At the end of the course surprises were always happy ones and students felt good about themselves and encouraged to continue.

Finally, let me note that strict scaling, making the course distribution into something like a normal curve is, IMO, always a mistake. It turns the course into a zero-sum game for students who can only win if someone else loses. In theory it should be possible for everyone to do well, even full marks, based only on their own work. Such scaling is also unjustified as it assume that a given sample (your students) perfectly match a population (all students). Statistically that is a serious error to make unless your scale is huge.