Since you are using PET bottles, you can safely let them carbonate longer. At room temperature, you need 60-80 psi to provide good carbonation once refrigerated. It might be worthwhile to make a cap with a bicycle valve so you can monitor one bottle. I have a Carbonator and a gas connector with a pressure gauge attached. I use it when I prime a keg mostly, but when I force carbonate a 2L bottle, I can check that as well.
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