USSR,
First, the good news:
I think you've come up with a really nifty technique that I've never heard of anywhere else. I intend to add it to my bag of tricks and recommend it to others as superior to using small base dies to size obstinate once-fired 7.62 machinegun brass. You are certainly right about that.
The bad news:
Bart is also correct. But I think that is probably the real secret behind why this technique works well.
So, before I go further, try this simple test. Take the decapping pin and expander out of your 30-06 die, turn it upside down and drop a .308 case in. Since the height of a shell holder deck is 1/8" above a case head, if the case shoulder stops against the die before the head is within 1/8" of the sizing die mouth, then it is running into the sides of the die.
I tried that with my Redding .30-06 FL die. First, with a fired Remington .308 Win case. Its shoulder stopped against the body taper of the .30-06 FL die with the head sticking out 0.9" beyond the die mouth. I then dropped in a brand new piece of Lapua .308 Win brass, and it stopped with the head sticking out a little over 0.6 inches. So, neither got anywhere near to having 1/8" or less sticking out. This means forcing those cases deeper into the die will narrow them to conform to the .30-06 body taper. The taper difference is gradual, so you will hardly notice the extra sizing effort it causes.
But there is good news in that. The reason a fattened 7.62 case won't resize down enough in a .308 Win FL die is that it is so much wider than a .308 sizing die that it has too much residual spring and pops back out too much after resizing; sometimes still too much larger to chamber in a rifle. By narrowing fat 7.62 cases in the .30-06 die, you succeed in eliminating the excessive diameter and then a little. It will spring out from the .30-06 size, too, but not enough to get back to .308 size, actually leaving it narrower than the .308 die is up much of the body and at the shoulder. If the shoulder is moved too far forward for your rifle's chamber by sizing this way, then you just run it through your .308 die one more time, and you would be good to go. That re-run through the .308 die will set the shoulder datum location while re-expanding the sides slightly. But because the sides start out narrower than the die rather than larger, this time they will spring inward upon withdrawal instead of outward. No doubt that makes the case easy to chamber. And, of course, despite being a little loose, it will fireform to your chamber at the next firing and never need either the .30-06 die or a .308 small base die again.