Edit: Tropical, this is long and if you don't want to read it all, just click on the video below.
The more detailed answer is that the phenomenon itself is complex and the term "throw angle for general usage" suffers from no definition of "general usage". What shot is that exactly? A loop? From close in or far away? Against a fast ball or a slow ball or against underspin? A block?
Different kinds of rubbers end up producing a higher throw angle in different ways. In some cases it is more topsheet-dependent, and in other cases it is more sponge-dependent. And with the topsheet, there is a contribution from the stickiness or grip of the top surface, and a contribution from how the underlying pips bend and rebound and of course some topsheets are thinner and more flexible than others. It's not like
I/kR or
I = V/R or
y = mx+b or some other simple relationship. Because there are so many parameters, and some of them interact in meaningful ways, you end up with phenomena like the throw angle being highly dependent on the velocity of the ball as it is moving towards the blade, and different rubbers having a different dependence on that. So you would need a bunch of different measurements and then you need complex graphical ways to express the relationships. There would be a bunch of nasty differential equations needed to model this.
And at the end of the day, who cares? Because most of the time,
you can adjust fairly easily, just like Airoc said.
Somebody was complaining somewhere that the throw angle of the rubber they were using was too low and they kept clipping the top of the net and wanted some other rubber to fix this problem. The answer? Just aim a little higher. (Especially since they were having this problem with
Tenergy 05!). I know that sounds simplistic but a lot of the time answers in table tennis come down to something like that. (Or time the ball better or move faster or see it earlier).
I have been playing since about 1969, and pretty constantly for the last 20 years. I have periodically changed from rubbers with different throw angles. It takes a few days to kind of adjust If the changes are not too great. It probably takes longer to become perfectly comfortable with it (which is a really good reason why trying to fix a technical problem by constantly changing rubbers tends to be counterproductive, and why really good players rarely change their setup).
At the end of the day, it is a cost-benefit analysis. Is the information worth the cost of doing it?
Of course some of the companies probably measure all this stuff for their own R&D, and have all sorts of knowledge about how changing this or that thing affects it, and it is probably all a proprietary secret. And why should they tell us information that lets us make more informed choices when they can sell more rubbers by getting people to try lots of different products in hopes of making some 0.05% improvement? The manufacturers love EJs, and that is why the introduce so many subtle variations on the same thing.
Actually, as a first approximation your approach would yield some reproducible information but it might not be all that helpful in predicting what it will be like when you actually play with it.
You can search some videos from a guy called PathfinderPro on youtube (he goes by Debater on some TT forums) and he has made some efforts to try to measure some of this with a highspeed camera. He is very smart and conscientious guy.
In fact, he is doing pretty much exactly what you propose.
And at the end he gives a long lost of reasons why his measurements may not mean much.
There is actually a set of three videos you can watch starting with this one:
In this one, he is trying to answer a simple question, does it make a difference what thickness rubber you use? You would think that would be easy to answer, right? Maybe not as easy as it sounds.
I really recommend it to you, it will give you really a better answer than this long rambling thing I just wrote.