This only partially acurate, because it has everything to do with how much you're spraying. My target A/F is at 11.2 when spraying for a little safety factor.
Let's just say you're target A/F is 12.8 when n/a and your car make 400hp and you spray 100hp shot, use the following equation.
[(400/500)x(12.8)]+[(100/500)x(7)]=11.64 new target A/F
Simple HP numbers and how they coordinate with the target A/F for air and nitrous.
This is accurate, and I'd also do what Gearhead talks about and learn to read the plugs. This is a science in it's self.
Also keep in mind that the cylinders will not all get even distribution on an LS1 without a directport, so make sure the safety factor is there. I personally like to add fuel to the injectors as well on a wet shot by about 10% to make sure there is at least a good distribution of fuel. Beleive me I've learned the hard way and have done my research.