You would use the "Full" distribution map. This map has 100x100 pixels, so setting a size of 2000' (20' x 100) in the Units parameter, the trees will be distributed in a grid, spaced exactly 20'. Same for other maps (all of them have 100x100 pixels), in this case will be some random gaps between trees.
Alternatively, if you need a linear distribution along a spline, use Tree Editor->Creation Tools->Along a Path, setting the distance in the Spacing parameter.