Radius of 850 mm after hemming, as it turned out.
What I did was stuff the raw square into the urn using a handy pail as a "plug" and adjusted this way and that until there were equal amounts hanging around all sides then went round with scissors to produce a sort of circle.
Spread the dodgy circle on the garage floor and drew a neater circle just inside the ragged edge with a felt pen, then cut that out. Came out fairly accurate, good enough for the job . (see OP)
What I did was stuff the raw square into the urn using a handy pail as a "plug" and adjusted this way and that until there were equal amounts hanging around all sides then went round with scissors to produce a sort of circle.
Spread the dodgy circle on the garage floor and drew a neater circle just inside the ragged edge with a felt pen, then cut that out. Came out fairly accurate, good enough for the job . (see OP)