Since the first post was written on the fly while I was working on the process, I wanted to sort of recap the process into more of a how-to explanation. I relied upon beersmith to do all the hard work, so unfortunately I can't give you all the awesome math behind the calculations in step 3.
Step 1: Come up with a realistic goal. If you're trying to make a 6% partigyle out of a 12% beer, you're not going to end up with much beer unless you are willing to add DME to the partigyle boil to up the OG. (Which, IMO defeats some of the purpose of the partigyle.) Rather, aim for 3-4% ABV and 2-3 gallons of beer (unless you are trying to make a table beer of 1-2%).
Step 2: Boil the wort down to 1.020-1.030 (depending on your goal) before you even start trying to calculate your recipe. If you are starting out with several gallons of 1.010 you're going to boil for a while before you get into a decent gravity.
Step 3: Once you have the wort down to an acceptable gravity, piece together a makeshift recipe by duplicating your original recipe, with the same grains, and reduce them down until you come up with a post-boil OG roughly .010 more than your pre-boil gravity (this may need to be adjusted based on your boil volume, boil time, equipment, etc.) at the pre-boil liquid volume. Then add your hops and other additions to get to your desired IBUs.
Step 4: Make beer!
Step 5: You may find yourself a little low on wort after the boil from what you were expecting. Don't worry, just top off until you get the right volume.
Step 6: Aerate, pitch, and ferment.
Step 7: Bottle/keg and enjoy.
Hope that helps!