Create responsive CSS form labels for centered navigation header - forms

Question

I am trying to make a responsive header/navigation form that has the following properties:
Has some type of label that is inline and vertically centered with the input box when the viewport is above a certain width.
At small viewport sizes, the label should center itself vertically above the input & button and the input/button combo should be 100% of the screen width.
Have a visually combined input and button, like the ones used in Bootstrap that won't split into separate lines
Either fill the width of the screen or be centered as the whole unit (header+input+button) when they're all on the same line
I have used this JS fiddle as a the starting point for the form to break the labels into a separate line on small widths and have successfully gotten it to work and for the label to center above the input at small viewports as well.
My code so far is
CSS
form > div {
clear: both;
overflow: hidden;
padding: 1px;
margin: 0 0 10px 0;
}
form > div > fieldset > div > div {
margin: 0 0 5px 0;
}
form > div > label,
legend {
width: 25%;
float: left;
padding-right: 10px;
text-align: right;
vertical-align: center;
}
form > div > div,
form > div > fieldset > div {
width: 75%;
float: right;
}
form > div > fieldset label {
font-size: 90%;
}
fieldset {
border: 0;
padding: 0;
}
#media (max-width: 600px) {
form > div {
margin: 0 0 15px 0;
}
form > div > label,
legend {
width: 100%;
float: none;
margin: 0 0 5px 0;
text-align: center;
}
form > div > div,
form > div > fieldset > div {
width: 100%;
float: none;
}
input[type=text],
input[type=numeric],
textarea,
select {
width: 100%;
}
}
HTML
1st Attempt
<header class="page-header row center-block">
<div class="center-xs">
<form class="main-form input-group" action="/caulcuate" method="post">
<label for="number" class="control-label"><h1 class="h1">BIG Label Header</h1></label>
<input type="number" name="number" min="10" max="6856" step="1" id="number" class="main-input"
placeholder="number goes here" required autofocus>
<span class="input-group-btn">
<button id="calculate" class="btn btn-lg btn-primary" type="submit">
100?
</button>
</span>
</form>
</div>
</header>
Attempt 2
<header class="page-header">
<form action="#">
<div>
<label class="desc" id="title1" for="numberhere"><h1>BIG Label Header</h1></label>
<div>
<input type="number" name="numberhere" min="10" max="6856" step="1" id="numberhere" class="main-input"
placeholder="number goes here" required autofocus>
<!--<span class="input-group-btn">-->
<button id="calculate" class="btn btn-lg btn-primary" type="submit">
100?
</button>
<!--</span>-->
</div>
</div>
</form>
</header>
Specific problems I'm having
In Attempt 2, I commented out the <span> that Bootstrap uses to group the button and input field because it will otherwise always put the button on a separate line.
In both attempts, the header label is not vertically centered with respect to the input box. I had it centered in an earlier iteration of Attempt 1, but I can't seem to get it back.
Attempt 1 just isn't centered properly at any size where the header row isn't taking up the full screen width. Attempt 2 does center consistently but the label header is too far from the input box at large screen sizes, so it's only centered in that there is roughly equal whitespace between the page border and the contents of the header.
Ideas for solution
Currently, the HTML in Attempt 1 seems like it is closer to what I want in terms of the label header and the button both being smooshed right up against the input box. Attempt 2, however, actually is responsive at small screen sizes.
It seems that maybe if I nest enough <div>s and am clever with their classes, I'll eventually get something that works about right, but would probably be fragile.
The other solution I thought of is to create two separate <header>s and wrap them in a pair of <div>s and use a #media query to change between which <div> is visible. This seems less fragile than using a ton of nested tags but also requires some code duplication. However, it seems like it might be the simplest option so far.
Somewhat related to switching between which <div> to display depending on the screensize, I suppose there probably is some jQuery way to do this and maybe CSS is currently the wrong tool for the heavy lifting in this case.
P.S. I wish I knew more about vector illustration so I could give an example of exactly what I'd want without all the descriptive text.