How To Adjust Scale Using Markers?

[jazzyj]

I have a project with photos taken using a UAC (DJI Phantom 4 Pro) and I put control points down that I just know the relative distances between them not absolute GPS coordinates.  The published workflow for Orthomosaics and DEM doesn't cover adjusting the scale based on identifying the marker distances.  I did identify two points and create a scale but I can't seem to check the box next to the scale (or the markers) and do anything with them. Trying to figure out what steps I'm missing?

[n@sk]

click the View Source Icon on the reference pane
double click on the Scale Bar Distance cell and input the distance that you have measured between the marker
click Refresh