使用double <ul>的CSS下拉菜单

Question

我正在做一个菜单，我正在尝试使用双<ul>下拉其中一个意见，菜单中的每个意见都是<li>。<登记/> The problem is that the second <ul> start where the menu start, and not where the <li> that I want to become drop-down-able is.

    header nav ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
}

header nav ul li {
    position: relative;
}

header nav ul li a {
    display: block;
    height: 100%;
    margin: 0px 12.5px 0px 0px;
    float: left;
}

header nav ul li ul:before {
    content: "";
    border-style: solid;
    border-width: 0 9px 9px 9px;
    border-color: transparent transparent #FF8002;
    position: absolute;
    left: 15px;
}

header nav ul li ul {
    position: absolute;
    display: block;
    margin-top: 48px;
    background-color: #FFF;
}

你能帮我解决问题或给我一些建议吗？