Yes it probably will be out. Even I've managed errors of 2 pixels or more. It's not the end of the world and can be fixed fairly easily. Larger errors really need a redraw or much heavier remedial work.
I must admit the odd conversion of FD scale has never made sense to me though!
Perhaps its advisable for the scales to made clearer somewhere so all newbies can find them easily and there is no mistake.
SB = 2 pixels per 1 foot (12 inches) (1 pixel = 6 inches)
FD = 22.093 pixels = 1 metre (1 pixel = 45.26mm)
English Electric Canberra FD
Interwar RN Capital Ships
Never-Were British Aircraft