Have you ever looked at ARP7488 Peening Design and Process Control Guidelines?
This is an SAE document that was extracted from AMS2430 several years ago. It was section 8.0 of AMS2430 the notes section. It was removed because of confusion that these are requirements and not just notes.
Something you'll notice here is they describe the intensity range differently. Example: Intensity, inch (mm) -0.002A (-0.05A) +0.005A (+0.13A). This is also how some other aerospace primes detail there engineering drawings. I much prefer it this way. So, if a drawing said 6A the range would be .004-.011A. This is considerably more practical.
Something else I thought about and failed to mention earlier with regard to the 30% rule. I believe this comes from an approximation. Create a process where you peen a test strip to intensity of .009A. Using a set up that the nozzle is a certain distance away from the test strip and maintains a 90 degree impact angle to the strip during the peening cycle. Now change only the nozzle angle to 45 degrees and repeat the process. The result should be close to .006A or approximately 30%. It's my belief that this is where the 30% came form many years ago.
AMS2430 also absorbed the old MIL-S-13165 years ago and only having a single digit intensity was common on older drawings, that's why this terminology may be in AMS2430 today. If it we up to me I would detail both the lower and upper limits on the engineering drawing always.