http://braukaiser.com/wiki/index.php?title=Understanding_Efficiency has a table showing theoretical first wort gravity for different mash thicknesses. I think this table is more accurate than the ones in the BYO article although they do complement each other.Thanks for the plug. I also have this one: http://braukaiser.com/wiki/index.php?title=Batch_Sparge_and_Party_Gyle_Simulator
I've used Kai's Batch Sparge and Parti gyle Simulator many times and it is very useful for predicting your gravity for a parti gyle, if you know your conversion efficiency. If you don't know your conversion efficiency, but know the sparge volumes and gravity from a previous batch, you can use it to reverse calculate your probable conversion efficiency, allowing you to use it predictively.
On brewday, you can use the gravity of the first beer runnings to calculate the exact gravity of the second. Since you know how much water is held up in the grain and the gravity of the wort, you can easily divide that gravity by the total volume of that wort and whatever water you wish to add. Adjust that added volume until you calculate the gravity you want for the second beer, then add that volume to your tun. The collected volume will be whatever you add, of course, so you might find you have to either make a smaller volume of beer than you intended or go with a lower gravity. Make sure you adjust the hops accordingly.
I don't think an IPA is a good candidate for a parti-gyle. The second runnings will have a very low gravity. If you are planning on doing a double IPA then you are in the ballpark, but you might need some malt extract anyway.
I think this is probably true if you try to make 5 gallons with just the second runnings, but you might be able to get 3 gallons of a low gravity stout.
Another option is to make a gallon or two extra of the first wort and add it to 4-5 gallons of second runnings, to bump up the second running gravity. This is basically how the Fuller's brewery makes their moderate gravity beers.