Using a #140 Block plane to make a shallow rebate on the back of the tail board is called the 140 trick. I'm sure it was done before Alan Peters used it but he popularized it.
The reasons the 140 Block plane works so well is two fold, the side plate is removable exposing the slewed cutter and it has a fence. The plane has a nicker as well which I do not normally use. Here is a photo of my 140 with the side plate removed and, I doubt you can see it, the cutter extended a thin red one pass the side of the plane.
I will normally use a TiteMark wheel gauge to mark the base line because the 140's fence will register on the board's end as does the TiteMark.
Setting the TiteMark. after gross setting I will give the micro-adjuster an 1/8 to 1/4 of a turn clockwise to deepen the baseline. I like the pins and tails to be slightly proud.
Next is deepening the base lines. I do this "freehand" with a marking knife.
Set the fence so the cutter just touches the near edge of the base line mark.
Take a couple or three passes until the rebate "looks about right".
Mark and cut your tails however you would normally cut tails. After the tails are cut, set the tail board on the pin board and aline the reference edges with a chisel. Mark your pins and if nothing moves and you saw and chisel correctly you should have a near perfect dovetail joint.
As always click 'em to big 'em.
I hope this helps with the 140 Trick,