Okay, I've updated it with some new features, and rectified the labelling.

First, I added the possibility of an offset for where the spring attaches to the lid, which probably more closely matches reality. There's a slider that allows you to adjust that.

Second, I added a slider to control the fully-open angle, in case something other than 65° makes more sense.

Now, my point for this is to allow you to move point B (where the spring attaches inside the box) around to see the ratio of the lengths of the spring when open and closed, which I've also added as a text callout, as well as offset distances of B from the axes.

What this does not do is deal with the amount of force required. This is just to help determine placement.

Also note that I'm not depicting the entire lid, only the portion that the spring directly interacts with.