Oh whoops sorry buddy - crossed wires there!
I actually still use plastic pots not bags, but what I currently do is mix up the Searles Premium Potting Mix Advanced (even though it has slow release fertiliser it doesn’t seem to give me any issues with Nute burn alongside the Aussie nutes brand I use) with perlite til it’s approx 1/3 perlite in one of those shell pools (I steal it off my dog! ) then I scoop the mixture into the pots and then give them a really good thump several times on the ground to pack it down. I don’t actually press the soil mixture down with my hands.
I used to fill the pots to the brim because it does still tend to pack down some more as the plants grow (with watering etc) but I found it a tad too full sometimes so now I leave an inch or so from the top.
I hope somewhere in that overly lengthy explanation is what you were chasing lol!