Hi Derek,
1) Distance from center of map is the correct approach unless you have location sensor on
2) We have found that Google changing some of their algorithms has messed up a lot of plugins that use the Google Map APIs, not just Store Locator Plus, everyone! The developer tackled a similar issue years ago with zoom adjustments. If you need immediate help or troubleshooting with him logging into your site we require the Login (Paid ) support option to open a support request. Otherwise we do not delve into a customer’s site by logging on as it opens up too many liabilities. P.S. When I tried to search your site url, it appears it is part of a large network not just one business and the site url you provided requires a login . I cannot inspect sites that require me to login. If you need a coupon for Premier (for use when you have multi-sites) please contact support.