Before Gyro Horizons had degrees of pitch added to their center display (as seen on contemporary Gyro Horizons etc), did pilots through operational experience and rough rule of thumb gauge their degree of pitch-down in level flight by observing where the horizon line sat relative to degree gradients on either side (see image)? To clarify, the red lines I added to the image would not be perfect 10-degree gradients as the horizon line moves up the display. As the numbered arc narrows inward and upward the 10-degree pitch values are likely closer to 9.8, 9.7 degrees. The Sperry Gyro Horizon pictured is a No. 643710 found in early model B-17, B-24, B-25 etc, later versions had a caging knob. I found one reference in the B-24 D pilot training manual as to best climb attitude which suggested "keeping the miniature airplane approximately 1/8th inch above the horizon bar" which roughly equates to 10 degrees or so. Has anyone read a similar statement in other WW2 pilot training manuals?