Carbonate using liquid post

Made an extract root beer and according to rebel brewer instructions set 30 psi for 6-8 hours and refrigerate.  Nothin.  Let it go for 24 hours at 30.  Nothin.  Hooked up to liquid out ball lock post at 30 psi and purged every minute for 5 minutes and it carbonated perfectly.    Wondering if the process translates.  Has any one tried this carb method for beer? Its the ultimate in hit and miss to get an exact carb level, but then so is setting your psi to 'X' and waiting for "x".

I use sankey kegs and this is how I do it normally. The concept is the same but I'm just switching the lines so that my gas is going in the beer out port.

My beer is starting at around 34ºF and I use 20psi for about 6 hours shaking fairly frequently. I end up with reasonably carbonated beer at the end of that time. It's not perfect and it's a bit variable but it does basically work.