It should be 30/70: Put the CO
2 value first, but that's only a convention so comes with minimal authority!
Reading this thread quickly I'm not sure the "physics" is getting across? Carbonating with CO
2 then N
2 isn't helpful. Each gas acts according to the proportion it is present ("Partial Pressure"), that means, carbonate at 35psi will result in just over 34psi of "carbonation" with nitrogen and 1psi with CO2. Well, not like it sounds, it's a mix. But the 30/70 adds up as 30psi + 70psi at
absolute pressure (excludes the weight of the atmosphere on the ground, or about 14.7psi).
What that amounts to is: Get 30/70 mixed gas as
@phildo79 is saying or else chase yourself into ever decreasing circles.
And Nitrogen is very hard to dissolve, so help it by getting the temperature down low. For a fairly long time. And perhaps do as Guinness and agitate at bone-crunching pressure (a "nitrogenater") to speed things along. You might get a special "stout" faucet to help get the nitrogen out again (into a head)
And nitrogen (and gases it is mixed with) remains a gas in the cylinders but compressed to a great degree (2-3000PSI). Pure Carbon Dioxide converts to a handy liquid at around 840psi so nothing like the pressure N
2 is delivered at (not a liquid without special cryogenic equipment).
Finally
(Hey, did I hear cheers?) you are
not attempting to make "Stout". You are attempting to make a freezing cold Guinness clone. Proper stout doesn't need "nitrogen".
And finally-finally
(Hey, did I hear boos?) apparently (!!!), nitrogen has an "anesthetic" effect on your ability to taste. That is responsible for the "smooth" flavour (and the low Carbonic Acid tang due to low CO
2 content).
(That said, I actually like the Guinness 0.0 "no alcohol" stuff!).