Responsive parallax height

Hi, what a great plugin, thank you. I need our parallax container to adjust according to device width, otherwise the content spills outside the container. I am thinking this could be achieved by way of a wildcard selector within a media query, using a calculated width.

For example:

section[class^="parallax_"] {
height: calc(<?php $custom_meta_fields('parallax_meta_height')?> * 2);
}

I haven’t been able to get it working though. Any pointers would be greatly appreciated ??
TIA, Bridie

https://www.ads-software.com/plugins/adamrob-parallax-scroll/