Using justify-content to space menu items evenly

Hi Guys,

I've been trying to use justify-content to space my menu items evenly without success currently they display with space between them but the first and last tems have no space in front or behind respectly.  I would like them to be evenly spaced with space in front and behibd the first and last so they sit nicely within the header/nav borders.  I've tried placing the code at different locations but the width 100% overrides.  When I remove it and do the same nothing happens.  Any help you can provide is appreciated.   Also any thoughts you have that could simplify the code are appreciated. 

<!DOCTYPE html>
<html lang="en" class="other">
<head>
<meta charset="UTF-8">
<title>CodePen - CSS tab menu v1</title>
<link rel="stylesheet" href="./style.css">
</head>
<body>
<nav>
  <div id="navigation">
    <ul id="mainMenu">
      <li class="home"><a href="#">
        <div class="icons"><span></span></div>
        Home</a></li>
      <li><a href="#">
        <div class="icons"><span></span></div>
        Products</a></li>
      <li><a href="#">
        <div class="icons"><span></span></div>
        Services</a></li>
      <li><a href="#">
        <div class="icons"><span></span></div>
        MENU 3</a></li>
      <li><a href="#">
        <div class="icons"><span></span></div>
        MENU 4</a></li>
      <li><a href="#">
        <div class="icons"><span></span></div>
        Contact Us</a></li>
    </ul>
  </div>
</nav>
</body>
</html>

* {
  margin: 0;
  padding: 0;
}

body {
  font-family: "Didact Gothic", sans-serif;
  position: relative;
}
a {
  text-decoration: none;
}
a, img {
  outline: none;
}
#navigation {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  text-align: center;
  font-size: 1vw;
}
#navigation * {
  transition: all 0.4s;
}
ul {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  text-align: center;
  list-style: none;
  display: flex;
}
ul li {
  flex: 1;
  background-color: #3E6EB6;
  border-radius: 10px;
  color: #FCF9F4;
  margin: 10px;
}
ul a {
  padding: 1em;
  display: block;
  color: #FCF9F4;
}
ul a:hover {
  background-color: #000080;
  border-radius: 10px;
}
ul ul {
  padding-top: 6em;
  height: 0;
  overflow: hidden;
  opacity: 0;
  display: none;
}
ul ul::before {
  position: absolute;
  left: 50%;
  top: 5.25em;
  margin-left: -0.6em;
  width: 1.4em;
  height: 1.4em;
  background-color: #000080;
  border-radius: 10px;
  content: "";
  transform: rotate(-45deg);
}
#navigation ul ul li {
  position: relative;
  width: 100%;
  display: none;
  background-color: #000080;
  border-radius: 10px;
}
ul ul li:nth-child(1) {
  font-size: 0;
  line-height: 0;
  height: 4px;
  display: block;
}
ul ul li:nth-child(even) {
  background-color: #000080;
  border-radius: 10px;
}
ul li.home:hover, ul ul li {
  background-color: #000080;
  border-radius: 10px;
}
ul ul a:hover {
  background-color: #3E6EB6;
  border-radius: 10px;
}
ul li:hover > ul {
  height: auto;
  opacity: 1;
}
.icons {
  position: relative;
  margin: 0 auto;
  padding: 0;
  overflow: hidden;
  display: block;
}
/*------------------------------
- TOP DOWN - ( PC -> Mobile )
------------------------------*/
@media screen and (max-width: 768px) {
  /* - Mobile Landscape & Tablet Portrait */
  #navigation ul li:nth-child(6) {
    display: none;
  }
  #navigation ul ul {
    display: block;
  }
  #navigation ul ul li:nth-child(6) {
    display: block;
  }
} /* /- Mobile Landscape & Tablet portrait */
@media screen and (max-width: 768px) and (orientation: landscape) {
  /* Mobile Landscape */
} /* /Mobile Landscape */
@media screen and (max-width: 560px) {
  #navigation ul li {
    width: 20%;
  }
  #navigation ul li:nth-child(5) {
    display: none;
  }
  #navigation ul li.home {
    width: 20%;
  }
  #navigation ul ul li:nth-child(5) {
    display: block;
  }
}
@media screen and (max-width: 440px) {
  #navigation ul li {
    width: 25%;
  }
  #navigation ul li:nth-child(4) {
    display: none;
  }
  #navigation ul ul li:nth-child(4) {
    display: block;
  }
}
@media screen and (max-width: 414px) {
  /* Mobile L Portrait */
} /* /Mobile L Portrait */
@media screen and (max-width: 360px) {
  #navigation ul li {
    width: 28%;
  }
}