title: Summarize all cells until a new entry appears
body: <p>For my personal budget calculations i have a long google spreadsheet which basically sums up monthly costs, etc.</p>
  
  <p>Obviously the amount of entries per month varies, so i want/need a formula which determines how long the list for the current month is, based on a label in an otherwise empty column on the side.</p>
  
  <p>Best explained with this example:</p>
  
  <pre><code>Jan 2017 | Testytestname | Sum
           | 200           | Calc_Sum_Formula_here -&gt; 500
           | 300           |
           |               |
  Feb 2017 | Testytestname | Sum
           | 350           | Calc_Sum_Formula_pasted_again -&gt; 450
           | 100           |
  </code></pre>
  
  <p>I tried to find fitting examples on the internet and here, but none were able to do this without messing up the layout severely (which i'd rather not do).</p>
  
  <p>Thanks for answers in advance :)</p>