That's taken from Lee's book chapters on obturation and ultimate compressive strength. Same information found on other sites. I believe Missouri Bullet has a tech section explaining the same.
The times .9 is just "reducing 10%". Same as 'give me 90%'. It's setting the load at 10% below the ultimate compressive strength of the bullet. Lee shows that as the ideal spot where accuracy is maximized. I don't see it as a hard and fast rule though, because many have found they can push pressure right up to the ultimate compressive strength of their alloy, and sometimes a bit beyond with good results.
Lee has that very cool example in his book where he walks the pressure up on several loads in a rifle, and then reports the accuracy. You see the accuracy improve right up before the ultimate compressive strength, and then fall off rapidly just after. Depending on the lube, alloy, bullet fit/design; I'm sure that can shift the results around.
For me, it's exactly what I've seen play out though; at least in my revolvers. I tumble-lube a thin coat of alox on all my bullets, even though they may be purchased and already have lube rings filled from the caster. When I cross over that compressive strength point, I start seeing leading follow the grooves in the barrel. Push it further, and I get a molten mess coating the barrel.
Now, with Laser Cast BHN 24 bullets I haven't yet found their limit. With other BHN 18 bullets purchased, I have found it...every time.