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.