I'd like to take this a step further:
Create a slider for digging and dropping. Slider is from .1 to 1.0 and indicates how much terrain elevation we want to change in one dig.
If you set your slider for 1.0 dig then you'll dig up 300 dirt in one go, or as much as you can carry.
If you set your slider for 1.0 dump then you'll dump 150 dirt in one go, or as much as you have.