How do I solve a wood shortage? Or: How can I safely get to some magma?

Due to the way caverns are generated, there's a fun little trick you can use to safely dig down to magma, most of the time:

If you are in a 4x4 embark, determine the exact midpoint of your area, and dig a 2x2 staircase straight down.

4 times out of 5, it will completely miss everything in your embark area, allowing you to access the sweet, sweet magma safely and without fun and lossage.

Building on Williham's answer...

I found that the Notes interface shows the (x, y, z) coordinates of the cursor's location, and (0, 0, z) is the center of the map. This makes it much easier to find where to dig a magma shaft on a 4x4 map.

Also, I dug a 2x2 shaft and happened to just clip the wall of the first cavern. Nothing Fun has come out of it yet, but I'll probably proceed with a 1x1 shaft from there, after I get some delicious wood, silk, and gems.