In general, however, draught Gunness is not a sour beer. It is quite a
fresh beer and has a smooth grainy taste with not too much roastiness and
a deep ruby red colour. Your O.G. of 1.039 is about right, and will give a
light-bodied beer (not the meal-in-a-glass that some people thing Guinness
is). Remember, it's drunk by the (Imperial) pint by men, women, and (most
likely) children all over Ireland as an everyday session beer.

The sour stuff is the "Foreign Export" variety, which comes in bottles.
This beer (or at least a portion of it) is aged in big old wooden tanks at
St James Gate which have a unique and diverse bacterial culture living in
them. This beer is also a lot higher in O.G. and, from memory (I haven't
had one in a while) a slight brown tint. This is also the version that
varies most in its brewed-under-license incarnation. The stuff made by
Carlton (Fosters Group) here in .au is 6% ABV, but they make one in
Africa (Zimbabwe?) that's even stronger -- somewhere around 8-9% IIRC.

Then, find the blackest roast barley you can find. As you've
noted, you really want the colour to be the deepest red, but you don't
want to have to put in so much roast that the beer tastes like coffee. I
think the 10% roast you used is a good amount.

Your 25% flaked barley seems like a lot, but its effect on the finished
beer will be a lot more subtle than any of the other factors, so feel free
to twiddle it generously in either direction to get the right balance of
maltiness, mouthfeel and head retention.

If you want to be authentic, hop with Target and Challenger. Your EKG
should be fine, but might leave a little hop character in the finished
beer. Personally, I like this, and often add a small dose (1/4-1/3 oz) of
finishing hops to my stout, but it's not really there in commercial
Guinness.

Lots of stout gets brewed, and it is often labeled as an "easy" style
because the colour and strong roast flavour mask a lot of flaws. This
might be true of the "sledge hammer" stouts that are brewed using every
type of roasted grain available, with some liquorice and coffee beans
thrown in for good measure, but I think brewing an excellent light-bodied
session stout is quite a challenge. It's certainly something that I
haven't achieved to my satisfaction yet.

Good luck, and happy brewing.