Create Custom Easing using Javascript

I am trying to create a custom easing function and I am experiencing a few difficulties. In other words, the maths is confusing me...

I would like my function to increment the width of a some elements within my page, and the increment value needs to decrease as the index of the element increases.

Consider these values:

50, 150, 230, 280, 310, 330

Whilst I appreciate it is likely to be impossible to generate these exact values, I essentially want a function that will come close to those values.

Here is what I have so far:

$('.something').each(function(i){
    var scale = Math.round(30*(i/10)*(2-(i/10)));
    dimension = scale*dimension;
    console.log(dimension);
});

This may be extremely easy but I am struggling to get my head around the maths today so any help would be much appreciated

Solution

Here's a live demo that shows how you can use a quadratic ease-out function to achieve a curve similar to the one you describe.

var out = document.getElementById("output");

// t: current time, b: beginning value, c: change in value, d: duration

function easeOutQuad(t, b, c, d) {
		return -c *(t/=d)*(t-2) + b;
}

for (var i = 0; i < 1; i+=0.1) {
  out.innerHTML += Math.round(easeOutQuad(i, 50, 280, 1)) + "<br>";
}

<div id="output"></div>

Shout-out to @c-smile for linking the jQuery easeOutQuad function in the comments, which this demo is based on.