Thank you, Eoin and Blaine, for your answers!
Say if I were to create a coach in a blender that is 20m long.
To find the scale length, I would do 20000mm × 0.0131 = 262 mm (26.2 cm)
As you had mentioned, Blaine the scale is 1:76.2. Would this mean that I should use 1:76.2 for the calculations, which (using the same 20m coach example) should be: 20000mm × 0.01312 = 262.4 mm (26.24cm)
Would this be correct? Again many thanks.