As otherothers have said: default to the syllabus grading scheme to the letter. Diverging from this sends you down a sinkhole of making more modifications on the fly, trying to be fair to all students, running different case what-ifs, and generally doing a lot more work.
Given enough heads-up (again, as otherothers said), I have specified a bare-minimum number of assignments that a student has to turn in to get a passing grade.
Also consider the appropriateness of an "Incomplete" grade. Although I almost never do it, if there is truly a unique situation that you want to account for, consider withholding the grade until the student passes in some bare-minimum work after the fact. Downside isA downside of this is that it does create more work for you (scheduling and following up), but in theory that grade status is designed to account for that.