I’m wrapping up this project now but still haven’t found a way to make this happen.
I have successfully managed to query custom fields dynamically using GET parameters by following the instructions at the bottom of this page: view-source:http://www.advancedcustomfields.com/resources/query-posts-custom-fields/
However, how can I return posts with a custom field between two values? IE: ?min-height=50&max-height=150
Many thanks 🙂