Front page map is zoomed in too far

With four devices at the following lat,lon, the front page map is centred on the correct location, but is zoomed in one notch too many so you can't actually see any of them!

51.24 -0.22
53.48 -2.25
53.79 -1.40
51.38 -0.10

In the "magic array" in html/pages/front/default.php, amending 6 => 8 to 6 => 7 fixes this for me, but I don't know about the general case.