While this suggestion has been posted before, my approach is a little different.

At skill level 5-25: You are able to terraform 1x 1x 1x

At skill 25-50: You are able to terraform maybe 2x 2x 2x or 3x 2x 2x etc.

At skill level 50-75: 4x 4x 4x

At skill level 75-100: 5x 5x 5x

Though you are able to terraform more ground infront of you, it requires more dirt to do so. You can still terraform 1x 1x 1x if you want so for 1 dirt.