Excel to Javascript web conversion

I have an Excel sheet calculation I'm trying to convert to a Javascript with web output. I'm rather new to Javascript but Im having some trouble with a function of the sheet I'm trying to implement.

The sheet is a Helium volume calculator that will take a given balloon type, multiply by a quantity then suggest the best value for which helium tanks are required to fill the given volume.

Here's the spreadsheet for reference:

http://ift.tt/1oHthjh

Here's my JSFiddle attempt at a short and simple web app:

http://ift.tt/1QlCpF7

var $ = document.getElementById.bind(document);

$('btn').addEventListener('click', function() {

  var

    tanks = {
      0: {
        name: 'HEL550',
        volume: 292
      },
      1: {
        name: 'HEL450',
        volume: 225
      },
      2: {
        name: 'HEL250',
        volume: 125
      },
      3: {
        name: 'HEL160',
        volume: 125
      },
      4: {
        name: 'HEL110',
        volume: 55
      },
      5: {
        name: 'HEL50',
        volume: 27.5
      },
      6: {
        name: 'BT 326502',
        volume: 14.9
      },

      7: {
        name: 'BT 306472',
        volume: 8.9
      }
    },

    balloonModel = $('balloon_model').value,
    qantity = $('qty').value,
    finalResult = balloonModel * qantity,
    finalResultRounded = finalResult.toFixed(2),
    tankCalc = tanks[7].name;

  $('result').value = finalResultRounded;
  $('tankyouneed').value = tankCalc;


}, false);

Basic volume calculation is easy enough and working. But I can't get my head around trying to use an array and loop to do the other stuff.

In the spreadsheet all the balloon types are listed but I really only need to chose a single one at a time, hence the dropdown. I don't need the tank calculations displayed, just which tank to chose as suggested by the spreadsheet result. To the right are 3 tank suggestions: actual tank(s) needed, maximum transportability or one-time use for those who don't want to ever buy again.

Can anyone help? Thanks!