Help with responsive menu layout

Hi, I'm an old webdev who is rusty in CSS; I learn about calc() recently and never really learnt display: flex properly.

I made some webs with a responsive menu layout (relevant CSS code posted on bottom). I tried using flex but I still had to do one ugly hack: I have the menu heights for the different resolutions hardcoded and I have to update them manually every time a new chapter is added. It's not a big deal, but I would like to know if there is a proper way to do this.

Some alternatives I've thought about:

• The new round(), but it's too new and not supported by most browsers.
• JavaScript

... but I feel like there must be a clean CSS-only way to achieve this.

Thanks!

Relevant CSS code (link to full stylesheet in case I missed something).

ul {
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
  height: 624px; /* =27x23+3 | 23 = 91/4 */
  margin: 0;
  padding: 16px 16px 4px 16px;
  vertical-align: top;
}
@media screen and (max-width: 1000px) {
  ul {
    height: 840px; /* =27x31+3 | 31 = 91/3 */
  }
}
@media screen and (max-width: 582px) {
  ul {
    height: 1245px; /* =27x46+3 | 46 = 91/2 */
  }
}
@media screen and (max-width: 400px) {
  ul {
    height: auto;
  }
}
  ul li {
    list-style-type: none;
    margin: 2px 16px 2px 4px;
    font-size: 120%;
  }
  ul li a {
    display: inline-block;
    background-color: #3fa79e;
    color: #d2e7e2;
    text-decoration: none;
    padding: 2px 8px;
    border: solid 1px #d2e7e2;
  }
    ul li a:first-child {
      width: 106px;
      margin-right: -3px;
    }
    ul li a:hover {
      background-color: #144c48;
      color: #fff;
      border: solid 1px #fff;
    }