For reasons that will remain undisclosed, I've become, recently, quite obsessed with jelly.
A part of my obsession led me to want to know the exact consistency that determines perfect jelly. So, several days ago, I conducted a little experiment.

Firstly, I looked at the amount of water set by the jelly packet. It says that it makes 500mL of jelly. Then I looked at the gelatin tin I had. One tablespoon (four teaspoons) sets 500mL of water.
So, it would be fair to say that there is one tablespoon of gelatin in the jelly packet.

I decided to set up 5 small cups of jelly, each with 100mL total. I would increase the amount of gelatin in each cup by 25%. So, cup one would have 4/5 teaspoons (four teaspoons in 500mL, so 4/5 teaspoons in 100mL), then each cup would have another 25% gelatin, until the final cup had 8/5 teaspoons.
I achieved this by making 250mL of normal jelly solution (50mL in each cup). Then I made 100mL of gelatin solution, with the same amount of gelatin in it as the jelly solution. I put 40mL of this into the last cup, and then 30mL, 20mL, 10mL and 0mL in each subsequent cup. I then filled the cup up to the final 100mL with cold water. This achieved the results I was looking for.

So, apprehensively, I labelled them and put them in the fridge.

...

Six hours go by...

...

I took out all five cups.
I sampled them in turn. Which was nice, because I like jelly. Anyway, the first cup was as normal, which makes sense, since it had 100% of the normal amount of gelatin (it was normal) - I refer to jelly as a percentage of the normal amount of gelatin that it has (so normal jelly is 100% jelly).
Anyway, the second cup was a little more nifty - a little firmer, but nothing special (this was 125% jelly)
The third cup was where I realised that I was onto something. It was pretty damn awesome. (150% jelly).
When I got to the fourth cup (175% jelly), I prodded it first. It was...quite interesting. It took a fair bit of force to actually leave a mark on the jelly.
I then ate it. it was awesome.

Then came the time. The 200% jelly. This was jelly that had twice the gelatin of your average, run-of-the-mill jelly.
I poked it too. I couldn't put my finger through it. Not until I angled it differently and my fingernail cut into it. It reminded me a little of ballistics gel. The most facinating thing I've ever experienced with my sense of touch. Very giving under my finger, but there reached a point where it gave no further, and I couldn't press past it. I ate it, and it was epic win.

So, my conclusion: use an extra tablespoon of gelatin when making jelly, for the win.