I do not employ the 100 point-anything-less-than-70-is-a-failure scale familiar to most students from their days in grade school. From a testing and grading standpoint, that scale really has nothing to recommend it except familiarity, and most people have different traditions about what grade constitutes an A, a B etc. This grading scale was evolved in an era before calculators had been invented, when all grade calculations had to be done manually and therefore had to be kept simple.

Working to an arbitrary percentage scale requires an instructor to make almost continual subjective judgements - every assignment, quiz, or other graded item must be designed beforehand to try to yield the desired level of numeric performance. That's an extremely difficult task, which is why so many instructors wind up applying arcane and often arbitrary "curves" afterwards.

Instead, I normalize all scores so that, no matter how easy/hard the assignment or how picky/lax the grading, the class's scores get mapped into a compatable range. This is essentially the same technqiue that is used on the SATs, ACTs, GREs and other national standardized tests.

This is not grading "on a curve". A "curve" is an ad hoc, after-the-fact adjustment made so that the numbers look nice on the traditional 0-100 scale. A "curve", by definition, can't be announced ahead of time. A curve is an admission by the instructor that their original grading policy has failed.