OpenLayers3 Custom Scale Select Options: Calculation needed
<p>I'm a newbie using OpenLayers 3 and having the following problem:
I've created a custom control (select box; see image) where I would like to add 3 predefined scale options (see code after image).</p>

<p><a href="https://i.stack.imgur.com/RcYWM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RcYWM.png" alt="image"></a></p>

<pre><code>let option2 = new Option('1 : 150.000', '2183910.7260319907', false, false);
option2.id = 'opt2';
select.appendChild(option2);

let option3 = new Option('1 : 30.000', '2183910.7260319907', false, false);
option3.id = 'opt3';
select.appendChild(option3);

let option4 = new Option('1 : 15.000', '2183910.7260319907', false, false);
option4.id = 'opt4';
select.appendChild(option4);
</code></pre>

<p>From other sources I've found that I "simply" need to pass the correct value (above it would be <strong>2183910.7260319907</strong> for the 1:2M scale) to the respective scale option and then I've already implemented the set resolution function in order to update the map.</p>

<p><strong>Question: How can I now calculate that particular value (I don't even know what it represents...) for the scales of 1:150.000, 1:30.000 and 1:15.000?</strong></p>

<p>Thank you so much in advance &amp; if something is not clear, I'm happy to clarify and provide more code/screenshots if necessary.</p>

<p>Cheers :)</p>