First off, do you have a stout faucet you are using? Nitrogen is pointless with a normal faucet. Secondly, don't carbonate to normal volumes. It should be somewhere around 1.4 volumes, pretty low. If you carbonate normally, you will get a glass of foam. On a balanced system, around 30psi at 38*F, with 1.4 volume CO2, and a Stout faucet, you should get a perfect pour. As for the gas, the 25% CO2 is needed to keep carbonation in the beer while the nitrogen is there to allow for a high pressure to push it through the restricter plate in the nozzle without overcarbonating the beer. So with 10% nitrogen, over time your beer will become less carbonated.