Here's my solution :)
Took me longer to get my head around this than I'd have liked, but once I figured out the principle then it went pretty smoothly.
72 metres away. Workflow runs in 16.1 seconds.
I first did it as large grid, then medium grid, then fine grid but, after seeing so many posts with the same approach, I tried using iterative macros to find a locale max after the first grid. Still a decent job at < 25 s.