JSFiddle - React, Tailwind, and code Playground

HTML

<form id="audience_details" class="form-horizontal" action="" novalidate>
  
<span class="clearfix">
  <div class="control-group">
        <div class="input-group input-group-lg pull-left">
          <input type="text" placeholder="Audience 1" name="audience_1_field" id="audience_1_field" class="field form-control" />
        </div><a rel="tooltip" type="button" class="link" class="btn btn-default" data-toggle="tooltip" data-placement="right" title="Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus leo est, rhoncus eget hendrerit eget, suscipit id urna. Integer eleifend imperdiet risus nec sagittis."><span class="glyphicon glyphicon-question-sign"></span></a>
      </div>
</span>


<span class="clearfix">
        <div class="input-group input-group-lg pull-left">
          <input type="text" placeholder="Audience 2" name="audience_2_field" id="audience_2_field" class="field form-control" />
        </div><a rel="tooltip" type="button" class="link" class="btn btn-default" data-toggle="tooltip" data-placement="right" title="Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus leo est, rhoncus eget hendrerit eget, suscipit id urna. Integer eleifend imperdiet risus nec sagittis."><span class="glyphicon glyphicon-question-sign"></span></a>
</span>


<span class="clearfix">
        <div class="input-group input-group-lg pull-left">
            <input type="text" placeholder="Total Reach" name="reach_field" id="reach_field" class="field form-control" />
        </div> 
            <a rel="tooltip" type="button" class="link" class="btn btn-default" data-toggle="tooltip" data-placement="right" title="Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus leo est, rhoncus eget hendrerit eget, suscipit id urna. Integer eleifend imperdiet risus nec sagittis.">
            
            <span class="glyphicon glyphicon-question-sign"></span>
           
            </a>
</span>


<span class="clearfix">


          <div...

JavaScript

function audience_details()

if(fields.length === 0) {

     var aud1, aud2, total;
     var aud1 = parseInt(document.getElementById("audience_1_field").value);
     var aud2 = parseInt(document.getElementById("audience_2_field").value);
     var reach = parseInt(document.getElementById("reach_field").value);

     var total = (aud1 + aud2 - reach ) / aud1 * 100 ;

      total_field.value =  parseFloat(total).toFixed(2) + '%';

      estimate.value =  parseFloat(total).toFixed(2) + '%';
}
else {
   //alert fields must be complete / contain numbers only!
}