The plans in the Longridge book are typically by Harold Underhill, not George Campbell. The plans in mine are printed very small as pages of the book. They have probably been reduced to a convenient size to fit the book page and are not printed to a set scale. In that case your best bet is to use a set of drafting dividers with the graphic scale.
I understand that some editions contain a larger folded set of plans. If that is what you have, lay a ruler next to the graphic scale and determine how many scale feet are in an inch. If they are printed to a standard scale I suspect that they are either 1/96 scale (1"=8' or 1/8"=1'), 1/128 scale (1"=10'-8" or 3/32"=1') or 1/192 scale (1"=16' or 1/16"=1'). Alternatively you can measure the distance between the ship's perpendiculars, which on Cutty Sark was 212'. That's 26-1/2" at 1/96, 19-7/8" at 1/128 and 13-1/4" at 1/192.