A few things to consider...

Unfortunately, the results of your test won't really provide meaningful information over the manufacturer's posted limit (assuming the rope is in good condition). Think about it. In the real world, how much tension do you really know you're putting on the line? Plus, on a real job, you're going to subject the rope to shock loading, the effect of which cannot be predicted by a slow load test. You can only make a rough guess, at best, about the effects of a shock load. Personally, I'm more concerned about breaks due to shock loading, rather than breaks due to slow loading. I figure a break due to a shock load is much more likely.

The physics of the rope materials has a lot to do with how the rope responds to a shock load. For example, nylon is quite stretchy. So, a nylon rope will be able to handle a shock load better then a hemp rope for example.

Also, if you bring your rope to its breaking limit, then you have weakened that rope. It's kind of like the Heisenberg uncertainty principle. The closer you get to understanding what you're testing, you inevitably CHANGE what you're testing. So, how does your test give you information about the rope sitting over there on your shelf? It really doesn't. The rope you just tested could have had a random imperfection, or perhaps something is not right with your testing setup. This goes back to the manufacturer's posted limit being the best rough estimate you have.