Is it acceptable evidence for the distance between lines to be estimated to calculate an alleged speed?

I passed 6 lines (at 2 metre spacing) in a 50 mph zone but was nowhere near the seventh. My alleged speed of 57 mph meant I must have passed the 6th line by 0.74 metres. I don't see how this could be measured with irrefutable accuracy because the photo is not calibrated to show this. Has anyone successfully challenged guestimates? Maybe there is a secondary detection radar that we don't know about. I have opted to challenge the evidence in court so someones previous experience could be invaluable. NB ACPO's guidence is for a fixed penalty at 57mph, 56 mph would mean passing the sixth line by 0.52metres so 22cms could make all the difference!