The answer is however many patches on the 4-ring circle are outside of the grass. Whatever this number is, use that as the numerator over a fraction of 80, the total number of grass patches possible, (9x9-1). That's the chance your chain will break when resetting or encountering on a border patch.
The reason a border patch is so dangerous is because the ring of patches directly around you is not guaranteed to produce a patch you can reset on, because that patch could spawn outside of the grass
