In Chile, we score from 1 to 7 (this is a legal normative), where 4 is the minimum grade to pass. Thus, the passing grade corresponds to halfway point (50%) of the grade span (from 1 to 7). However, it is common practice to require a minimum score of 60% to pass (e.g. 6 points of 10). This creates a dissonance between the minimum grade and the minimum score to pass.
If we use a minimum score of 50% (4) to pass, we use a linear function to assign grades. However, if we use a minimum score of 60%, we need two functions to calculate grades from scores. In other words, the function breaks at the minimum grade to pass, such that points at the bottom (below 4.0) are worth less than the points at the top (above 4.0).
Having two functions to assess the performance troubles me. In other words, people that have more troubles in the course are being evaluated with harder standards than the ones with better performance.
One solution to this problem would be to change the grading scale. Unfortunately, this is not legally an option in the short term. A second solution would be just to drop the minimum passing score of 60%. But I think sometimes you may need to increase the grading difficulty because of the importance of the content you are teaching.
Under those circumstances, what is the better solution in this case: a single linear function or two different functions to grade scores?