hello!! i am SO sorry for how long it took me to get back to you – i rarely check notifs on this journal so i just totally missed your comment, my bad!! here is a fix for you:

<lj-raw><div style="margin: 0 auto; width: 400px; display: flex;"> <div style="width: 250px; height: 400px; background: url('https://i.imgur.com/EdayhGQ.png') center; background-size: cover;"></div> <div style="width: 150px; height: 400px; background-color: #000000;"></div></div> <div style="margin: -300px auto 0 auto; width: 400px; height: 400px; font-family: georgia; font-style: italic; font-weight: bold; font-size: 13px; letter-spacing: 3px; display: flex; align-items: center; line-height: 200%; text-align: left;"> <div style="width: 400px; margin-left: 233px;"> <a href="LINK_URL_HERE" style="text-decoration: none; padding: 2px 2px 2px 2px; border-left: 17px solid #000000; background-color: #fff; color: #911c35;">link one</a><br /> <a href="LINK_URL_HERE" style="text-decoration: none; padding: 2px 2px 2px 2px; border-left: 17px solid #000000; background-color: #fff; color: #911c35;">link two</a><br /> <a href="LINK_URL_HERE" style="text-decoration: none; padding: 2px 2px 2px 2px; border-left: 17px solid #000000; background-color: #fff; color: #911c35;">link three</a><br /> <a href="LINK_URL_HERE" style="text-decoration: none; padding: 2px 2px 2px 2px; border-left: 17px solid #000000; background-color: #fff; color: #911c35;">link four</a><br /> <a href="LINK_URL_HERE" style="text-decoration: none; padding: 2px 2px 2px 2px; border-left: 17px solid #000000; background-color: #fff; color: #911c35;">link five</a><br /> <a href="LINK_URL_HERE" style="text-decoration: none; padding: 2px 2px 2px 2px; border-left: 17px solid #000000; background-color: #fff; color: #911c35;">link six</a> </div> </div> <div style="margin: 3px auto; width: 400px; text-align: right; font-family: georgia; font-size: 9px; letter-spacing: 4px;"><a style ="color: #888; text-decoration: none;" href="https://withnails.dreamwidth.org/5917.html">CODE</a></div></lj-raw>

basically you were almost there, it's just that from my (limited, lol) understanding of html, auto margin sort of centres the div by pushing it in equally on both sides based on the width of the display, so if you have the margin set to auto on the right but 40px on the left it's going to be different on every display. i hope this is what you were looking for!!