It is easier to use a slide rule with two tan scales when doing the Bygrave computation but if you only have one then there is a technique to use.
The first photo shows my Pickett N 3-T that has the double tan scales. The next shows the cursor on the 76 degree mark on the second tan scale and the tan of 76 shown on the "C" scale at 4.01.
K&E 1 shows my K&E 4080 and K&E 2 shows the cursor on the 76 degree mark on the reversed tan scale. K&E 3 shows the slide rule turned over with the cursor now lined up on the "C!" scale at 4.01, the same value found on the Pickett.
This is the way Bob did it and then he had to transfer that value of 4.01 to the "C" or "D" scale, an extra step which intoduces an additional source of inaccuracy and possible error.
Now here's the trick.
K&E 4 shows the cursor set on the left index of the "D" scale with the 76 degree line on the tan scale lined up with the cursor.
K&E 5 Then the cursor is moved to the right index on the "C" scale and the tan of 76 degrees is now found on the "D" scale. Then proceed as normal. No need to transfer the value from the "CI" scale to the "D" scale since it is already there.