Well, if cheating is allowed, you'd want to find some way to compress more information into this. For example, you could somehow take advantage of the medium of sentence delivery.
"Please divide the length of this sentence by the length of the stone block it is written on and convert to base 26 in order to reveal a code which can be cyphered using the obvious number-to-letter mapping"
If this works like I think it will, you should now be able to convey an extremely large amount of information in plain english - constraints being set by the maximum length of the stone block and the accuracy of your measuring and cutting tools.
The best thing about this is that more of the message gets revealed as the society's measuring tools get more accurate...so you can time the release of information, putting more advanced or potentially dangerous information closer to the end. The downside is that if anything happens to the original stone before humanity 2.0 achieves the required measuring proficiency, the society loses any undecoded messages.
"Please divide the length of this sentence by the length of the stone block it is written on and convert to base 26 in order to reveal a code which can be cyphered using the obvious number-to-letter mapping"
I don't think it would be feasible to encode even half a dozen letters with that technique.
So apparently Richard Feynman once said:
I could be missing something, but this strikes me as a terrible answer.
When was the atomic hypothesis confirmed? If I recall correctly, it was only when chemists started noticing that the outputs of chemical reactions tended to factorize a certain way, which is to say that it took millennia after Democritus to get the point where the atomic hypothesis started making clearly relevant experimental predictions.
How about, "Stop trying to sound wise and come up with theories that make precise predictions about things you can measure in numbers."
I noticed this on Marginal Revolution, so I shall also state my candidate for the one most important sentence about macroeconomics: "You can't eat gold, so figure out how the heck money is relevant to making countries actually produce more or less food." This is a pretty large advance on how kings used to think before economics. I mean, Scott Sumner is usually pretty savvy (so is Richard Feynman btw) but his instruction to try to understand money is likely to fall on deaf ears, if it's just that one sentence. Think about money? Everyone wants more money! Yay, money! Let's build more gold mines! And "In the short run, governments are not households"? Really, Prof. Cowen, that's what you'd pass on to the next generation as they climb up from the radioactive soil?
*Cough.* Okay, I'm done. Does anyone want to take their own shot at doing better than Feynman did for their own discipline?