Here’s what I do. I force carbonate @ 35psi for 2-3 days. Bleed the pressure from the keg then serve @ 7-11 psi. One thing I’ve noticed is warm liquid doesn’t seem to carbonate. I don’t have a kegerator, so I put mine outside so it cools a bit.
YMMV
The best extract for force carbonating or dry ice root beer is Hires Big H. It isn’t cheap but it’s fail proof. You can get it on amazon or their website. It’s cheaper on amazon unless you buy 3+ bottles at a time. Zatarains is good if you yeast carbonate for a few days then let it set for about...
I’ve made this before and bottled it. I had to let it set for two weeks before I could stand to drink it. After two weeks it was pretty good. I still prefer Zatarain’s.
I use old beer bottles and soda bottles. A rag tag mix of whatever beer bottles my buddy gave me and some Pop Shoppe bottles we had hanging around since the early 80’s.