I usually do NOT drop an exam grade because that method has caused problems in the past.  I take 50% of the lowest exam to minimize the effect of one poor grade.  Instead of entering a number to signify how many grades are dropped in a given category, a formula would be allowed.  In order to drop a grade, the category must be able to identify the minimum score in a category.  To drop more than one, a loop must be used to drop other low scores. 

Instead of dropping the low score, only 50% of the low score would be dropped along with 50% of the total exam score.  The new average would provide the new weighted average taking 50% of the lowest exam.