"... So note the position of the nose wrt the background, then when the tail reaches that point wrt the background note the new position of the nose and repeat etc . ..."
" ... measuring the distance of the background adjacent to the tail at the begining of the last second to the position of the background adjacent to the tail against the at the end of the last second, measured as a proportion of the distance between the pylons and then applying your scale factor? If so that doesn't work because the background distance is not all at the same distance ... "
I made sure that I established my reference points and then calculated scale. How did you establish that in your method, as you have stated you used the background AnFI?