Partigyle: 2/3 & 1/3
I am looking for some more info on partigyle brewing. I get the process and I have seen some great charts/calculators for 1/3 & 2/3 batches, and for 1/2 & 1/2. What I am looking for is how to calculate for 2/3 big beer, 1/3 small beer. I brew 10 gallon all grain normally, so I would do this as 10 gallons of the bigger one and 5 of the smaller one. I have been trying to figure this out  damn logarithmic sugars  and am stuck, so thanks ahead of time.

Not sure how much I can help because I've only done one partigyle and it was just last weekend. I also did two 5 gal batches but one with an OG of 1.082 and the other at 1.048. And I 'cheated'.;) But here's what I did.
Basically I had plenty of DME onhand (by design, I needed to use up a whole 3# bag before it got old). When lautering/sparging, each of the two boil kettles were filled to about a gallon LESS than the preboil volume I needed. Then I just measured the gravity and exact volume of each kettle, did a little math, then added the required amounts of DME and water to hit my target preboil gravity and volume in each kettle. Then boiled each one normally from there. I went way smoother than I expected and I hit my volumes/gravities. I also did a decoction mash and ran out of propane midboil...made for a 9 hour brewday. But I like brewing so it was fun. And here is Mosher's old BT article on it. HTH and good luck! 
One thing you could do would be to collect your big beer as a 10 gallon nosparge batch and your small beer from a batch sparge. You'll need to know your conversion efficiency for this. Once you've calculated the volume V and gravity G you need to collect to hit your desired original gravity for the big beer, you can calculate the necessary mash thickness by first converting G to degrees Plato (call this g), then:
Mash thickness (qts/lb) = .48((qts/lb)/(l/kg))*(extraction %/100)*((100/g)  1) If you are getting 100% conversion, your extraction % will just be the extract yield percent of your grain, typically around 80%. Now you can calculate your grain bill weight: M = Grain weight = (V in qts)/(mash thickness  absorption rate in qts/lb) Absorption is typically around .1 gal/lb or .4 qts/lb. Now you can see that this is maybe not such a great idea: Using this method, assuming 100% conversion and 80% extraction, if we want to collect 12 gallons of 1.092 SG (22 Plato) wort to boil down for a big beer, we will need a mash thickness of 1.36 qts/lb, which translates to 50 lbs. of grain. That's an awful lot, and the calculations for the small beer are even more complex... 
Thats the split I usually do. At a mash thickness of ~1.3 qt/lb, I use my 2nd runnings for the small beer (~2025% efficiency) and 1st & 3rd (~65% eff) for the big beer. You can use kai's batch sparge simulator to test the correct volumes.
http://braukaiser.com/wiki/index.php/Batch_Sparge_and_Party_Gyle_Simulator 
i've done a few and frankly i don't like them... they're always missing body and complexity.. the best ones have been no sparge first beer (or minimum)...
hop it lighter than you otherwise would because maybe it's just me, but there's a lot more natural bitterness in a partigyle brew imo. if you come up with something that translates body, let me know... i've added a small amount of DME at the end of the boil to add body back in, helps a bit. 
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