You could of course work in reverse.

First heighten the area to the level you want it to be,
then go down and start mining out the trench around your base.
As long as you keep either the left or the right side of your character next to your base, you won't go wrong.

And if it helps you more, you can always use the wooden beams to give you a sense of direction. They do clip through the ground after you have placed them, so place for instance a log beam paralel to what you've dug out, connect it to a 2nd log beam and you got at least 4 meters you can continue mining out in a straight line.

Thorin :)