big_supper's recipe, albeit simplistic, is exactly right for a Guinness clone... Add White Labs Irish Ale Yeast and you got pretty much everything.
For Guinness:
70% Marris Otter
20% Flaked Barley
10% Roasted Barley
OG should be 1.038 and FG 1.006 (!!)
IBU, around 40.. Single addition at 60 since there is no hop taste/aroma.
I tried this recipe many times, with many variations. The above malt bill gives excellent results, and it seems to be the consensus for the Guinness draft recipe. Also, Roasted Barley is *the* irish stout malt. Forget the black patent or chocolate malt, not needed at all... Same for the crystal.
For a partial mash, I would calculate the amount of malts like if it was All Grain, and then remove only a part of the Pale Malt, and replace it with the calculated amount of extract that will provide the same specific gravity...
The problem is that this recipe calls for 20% flaked barley.. if you reduce the amount of pale malt in the partial mash by half, that means you will have around 40% flaked barley.. That would be the maximum, otherwise conversion will be difficult (flaked barkey has no enzyme)... So, you might want to reduce the amount of flaked barley.. which should not be too dramatic..
I would suggest concentrating on the above for a first try, but brewing a dry stout like guinness has some real challenge that you might want to tackle later on:
First, Roasted Barley will acidify your mash and might give a more acrid taste to the final beer. Even if Guinness has a nice tang to it, it is not acrid, mostly because the hard Dublin water is good for dark beers and buffers the acidity of the roasted barley... After many failed attempt to correct this problem in my dry stout, I came up with the solution of adding a teaspoon of Calcium Carbonate (CaCO3) and baking soda in the mash (I do 6 gallon recipes). You will need to do your own trial and error here to see what works for you..
Second, the target OG of 1.006 is not that easy to reach. Make a bigger than normal starter and aerate wort as best as you can. Since most of your fermentable come from malt extract, try finding the most "fermentable" extract.. some extracts are more fermentable than others.. If you would be doing a full mash, keeping the temperature at the lower end of the scale and doing a long mash might help fermentability. Still, I have never reached the 1.006 mark.. My dry stout always finish at around 1.010..
Finally, the famous Guinness twang... I havn't actually got around to tackling this one
My next attempt will be setting aside about 1 or 2 cup of fresh wort and letting it spoil by adding a few unpasteurized malt grains in it... Then after a few days, boil it and add it back to the secondary.