Button holes placement help

In my older post, I made a button down shirt for my daughter here. I was looking for a tool to help me with equidistant button hole placement. I found it.  It is called the Simflex Expanding Gauge, which sounds like it has nothing to do with button holes or sewing.  But in reality, it is made for this purpose, as well as for other measuring needs.

For a closer look,  I have taken four pictures showing the gauge with finer details.

Picture 1 shows the Simflex out of the box. Picture 2 shows the Simflex when stretched like an accordion. The pointed tip has markings for the button hole length up to 1 inch (picture 3).  The rounded tip at the other end has markings up to l 3/4 inches (picture 4).

How about marking the placement for the buttons?  Well, a tip that professional seamstresses use in the factory is to first make the button holes, and then line up the finished side of the holes with the side for the buttons.  For each hole, mark a dot where you want the button to be, usually at the center, then sew the buttons.  This is a smarter way of doing it, because sometimes the holes are not sewn exactly according to its placement , and its alignment may be off from the button markings.  It's not possible to undo a button hole, so once the cloth is cut, it's cut. Therefore this is the method that is used in the factories.