Let's take a look at the scatter plot below. This is a chart of the grades from geometry classes from two high schools in a moderate/large district. As one can see there is a clear relationship between how students did on the HW and how they did on the major assessments. The scores on the summative assessments is represented by the x-axis, while the score on the HW is represented by the Y-axis. One would expect this, but if we pick a particular pont on the x-axis we can see there is a range of performacne on the HWs. Lets take the summative grade of 60%, barely passing. Some students scored 100% on the HW. Some scored 40%. That would mean that in these high schools that have a grade weighting policy of 60% for summative assessments and 40% for formatives (in this case mostly HW), that would give student 1 an F(52%), student 2 a C(76%) and student 3 a B(92%) depending on where the cut off was. Jefferson County, the biggest district in the state of KY had a percentage cut off of 93% for an A up until recently.
You have to decide is that grade meaningful? What do your grading policies result in? Is it what you intend? Does student 2 really deserve to get 2 letter grades higher than student 1 even though their summative average is less than 60%?