From what I've picked up, and learnt the hard way, you're looking for about half the volumes with beer gas for stouts and English ales. So if the beer blend is 25% CO2, then you're looking for double the pressure to get the CO2 partial pressure down.
For my system, I run CO2 at 14psi for regular beers, and hold my nitro beers at 28psi, which puts the CO2 partial pressure at 7psi. You also need nitro beers at 25-30psi to get the right flow through the faucets. Line length isn't so important on beer blend.
Now you can carb on straight CO2 to the volumes needed then just transfer over to blend, or if you're patient, just hook up to blend and forget it, but it takes a while longer to carb up (even to half the volumes) since the headspace becomes saturated with N2 as the CO2 dissolves.