responsive header image

I have a header
.siteheader {
      height : 155px;
      width : 100%;
      max-width: 1200px;
      margin:0 auto;
      background-repeat : no-Repeat;
      background-image : url("../images/header1.png");
      background-position : 0px 0px;
      background-size: 100% auto !important;
      /*border-bottom:1px solid #999;*/
}
as you can see I have the image resize according to browser size

I REALLY want this border-bottom but have commented it out because as the image height decreases then I have empty blank space within the header because the header itself is not resizing - so removing the border looks like the empty space belongs to the content area below the header.

question is how can I resize the header as the browser window changes size
I can then probably use background-size 100% 100%
I've tried
.siteheader {
      max-height : 155px;
      height : auto;
      min-height : ???;
what would i put for min-height?

or actually what should I do here with this situation?
thanks