Here's the 1968 service manual, since you have a 68.
http://67-72chevytrucks.com/vboard/s....php?p=5781194
The link you gave is for this tread that we are on now.
Something much like that drawing is in the 68 service manual on page 5-10. The text that goes along with is on page 5-9. Figure 18 is for G-10-20 and Figure 19 is for our trucks (C-10-20).
It is interesting the drawing you posted gives more details about manual and power brake distances.
The drawing seems to vary from year to year a little, and I don't see it in the 1970 manual.
I don't think it's something you adjust, it is an observation you make and if you observe too little clearance between the pedal and the floor you should investigate what's going on with that. That would be measured with 90 lbs applied to the brake, and would be after making the free pedal travel adjustment on the push rod, and after a fresh adjustment on the brakes with drums and other parts that aren't worn.
So, I'm thinking what puts the pedal at that height in the picture is 90 lbs of pressure applied to the brake pedal, assuming a properly adjusted stock system in good repair.