Responsive landing page primarily employing flexbox

Creating an order or routine for establishing changes based on media queries was a stumbling point. Any guidelines for best practices for setting CSS with media queries?

iOS for iPhone in landscape mode thinks body height is the physical screen dimensions. Best practices for layouts in phone landscape? My inclination (I have not yet followed through on this) is to create a media query and CSS rule-set for phones in landscape mode, specifically to fit the screen without needing to scroll