Scaling a fixed button

Hi guys,
I am using the following code to isolate a button at the bottom of my screen. My problem is that setting the button as “fixed” wrecks the formatting in desktop display. I’ve tried sticky too and that’s not working either. Any ideas on how to make the button scale to the right width while also keeping it isolated as the user scrolls?

div[id*='screenScrollView'] > div > :nth-of-type(11) button {
position: fixed;
bottom: -30px;
width: 91%;
    background-color: black;
    margin-bottom: 80px;
    color: white;
    border: 1px solid #B3B3B3;
box-shadow: 0px 0px 10px 3px rgba(199,197,199,1);
    animation: fadeInUp 2500ms;

The 100% width in desktop and mobile are different. I think the best thing you can do here is use an external code column to know whether the user is using mobile/tablet/desktop and put in the right snippet of code to use, in an if-then-else column.