I know this is an old question, but nowadays there is a super easy form to do that, which is CCS Grid, so let me put the divs as example:
<div id="full">
<div id="header">Contents of 1</div>
<div id="someid">Contents of 2</div>
</div>
then the CSS code:
.full{
width:/*the width you need*/;
height:/*the height you need*/;
display:grid;
grid-template-rows: minmax(100px,auto) 1fr;
}
And that's it, the second row, scilicet, the someide, will take the rest of the height because of the property 1fr, and the first div will have a min of 100px and a max of whatever it requires.
I must say CSS has advanced a lot to make easier programmers lives.