My understanding of Eratosthenes experiment is that he was aiming to calculate the circumference of the Earth, not the diameter. Considering how long ago he lived I would say he did a pretty good job.
FE Wiki states...
It's a common misconception that Eratosthenes was measuring the circumference of the round earth in his shadow experiment. Eratosthenes had simply assumed that the earth was a sphere in his experiment, based on the work of Aristotle. He was actually measuring the diameter of the flat earth (distance across), which is a figure identical to the circumference of the round earth (distance around).
If that is true then why is it that the only reference I can find to this claim comes from websites relating to Flat Earth believers? I have checked many other sources which all state that Eratosthenes was assuming that the Earth was a perfect sphere. It was common in ancient times for philosophers, scientists, theologists etc to believe that the Sun, Moon and Earth were perfect spheres and that the Earth was at the centre of the Universe. The sphere was seen as the most perfect of the solids and so it natural to believe that the Earth was a sphere. Close but not quite right.
Eratosthenes realised the Sun was very distant so he also made the assumption that the Suns rays are parallel. So why should the notion that he was measuring a flat surface have entered his mind at any stage? For his experiment to work the way it did if the Earth was flat, Eratosthenes would have had to have been measuring around the edge of the Earth. And that, as the FES states, is somewhere that no one has ever been. It is of course easy to create ones own interpretation of an experiment to suit ones own beliefs.