Sorta. The stade was based on the pous which varied. But not that much, and in ways that are often consistently documented. Around the time Eratosthenes was alive, give or take a couple hundred years, it was documented that 1 Roman mile = 8 stades, which gives us something to go of off. While there are other possible definitions, we do have one that we know is probably closest to whatever Eratosthenes used. Using that definition, he had a 15% error. Depending on how you define the stadia, the error can go as high as 30%. I’m still impressed he got within the right power of ten with nothing but a stick.

maa.org/…/eratosthenes-and-the-mystery-of-the-sta…