Each time a calculation is added to the gradebook, the min and max values for that new grade_item default to 0 and 100.
This causes that, for a lot of calculations, the teacher has to edit the new grade_item in order to move those min and max values to get proper percentages and so on.
I would propose (alternatively):
1) Automatically calculate those min and max values for the new grade_item (by applying the formula to the min and max original values). Don't forbid ulterior editing.
2) Decide, with one check in the formula editing, if that min/max calculation must be performed.
Note that some formulas can present problems, so situations like max < min should be checked, perhaps defaulting to the default 0...100 interval (and potentially warning to the teacher about that).
But most "increasing" formulas can use any of the approaches above saving some clicks and headaches to teachers by default.