I'll see if I can run the numbers, assuming that the rules you cited are correct.
Quote:
Originally Posted by bighorn_brew
"For instance, 5 gallons of beer at 1.090 and 5 gallons at 1.060 require (5 X 90)/(5 X 60) = 750 points. Dividing that by the total volume (10) points to a mash for 10 gallons at 1.075. His rule of thumb declares the first third of the runnings contain half of the sugar (e.g. 3 gallons at 1.125)."
Question 1. If I were to take that first third runnings of beer, and dilute to 5 gallons what would be my estimated specific gravity.

By your rule, you'll get half of the 750 points375 pointsin your first third of the wort. You then dilute to 5 gallons, so you have 375/5 = 75 ppg, or 1.075 SG. (The other way to see this is you're taking half the sugar from your 10 gallon batch and mixing it in half the volume, so it'll be the same SG.)
Quote:
Originally Posted by bighorn_brew
Question 2. If I were to take the last two thirds (7 gallons of preboil) of the runnings, what would be the estimated preboil gravity, and post boil at 6 gallons?

You now have the other 375 points in 7 gallons, or SG=1.054. When you boil that down to 6 gallons, it's 1.063.
Quote:
Originally Posted by bighorn_brew
Question 3. What would be the specific gravity for the first third runnings (4 gallons). And secondly, what would be the gravity of the second 2/3's runnings.

Well, calculate the total points in the recipe and follow the method above. If you're going to dilute/concentrate them to samesized batches, then they'll both be the same SG since you have equal points in both.
If you take them at the original concentrations, then the first runnings will have twice the SG of the second (equal amounts of sugar, but twice the volume in the second runnings by your method).
Quote:
Originally Posted by bighorn_brew
Question 4. I want to make an strong golden ale, then add in the crystal, chocolate, and carapils malt. If I left out those grains, drew the first runnings off, then add grains to mash, stir, etc, would the second 8 gallons approximate a porter, and if not would I need to add anything else to the mash (e.g. more pale malt).

I imagine this would work, but I don't have experience doing (or even thinking about) anything like this.