I'm GUESSING that the 'bearing area' refers to the proportion of solid material to air at 1/4 of the Z depth of the surface, referring to a traditional profilometer plot. I.e., 'just' draw a line at 1/4xRz on the profile plot, then measure the axial length of the plot that's above the line and the length that's below the line, and report the ratio as a percentage.
One problem with that conjectural interpretation is that the result is not really a percentage of area, since you would be working with what is effectively a cross section profile of the surface, so the result would be a percentage of lengths.
One other problem is that the instrument you are using does not produce a profile plot.
Maybe it does so internally, and does all the measuring internally, and reports the measurement you seek as a number, but I can't tell.
... because you haven't even done us the simple courtesy of identifying the instrument you are using, so we could seek out the manual and read it for ourselves.
Mike Halloran
Pembroke Pines, FL, USA