I always use decanted Tamiya primer before paint of any kind goes on a model. It helps keep the fresh paint from being pushed around on the model while you're airbrushing.
Here is the mess free, relatively fume free method I use for decanting it indoors...I live in Colorado where both wind and cold have made doing any painting outside something that can be done...but only on rare occasions. I have a few posts in the thread in this link that go into detail of how to do it, along with pictures of the steps.
https://cs.finescale.com/fsm/tools_techniques_and_reference_materials/f/18/t/189063.aspx
I also use a Paasche H, and when your paint is thinned properly, you should be spraying at no more than 20 PSI. With that said, you definitely have the right plan for practicing on plastic cups (I use plastic spoons for when I want to try a new brand of paint I haven't shot before, or when I want to see what a color will actually look like when its cured on a piece of plastic). Try not to get too hung up on the techniques of guys on YouTube. One thing about those videos is that you, as a viewer, can't actually see much detail...nor do you know what the video maker thinks is a "good paintjob". I've seen plenty of close-ups of "good paintjobs" posted in various places on the internet, and a lot of them look to me like they were painted in a sandstorm...usually in a fuzz-covered paint booth. What's important is to gather as much info as you can from various sources, practice on spoons, and see what works best for you. Personally, I cringe when I watch those videos where the guy gets complete coverage on one or two passes...they are shooting WAY TOO MUCH PAINT in my opinion.
As for how much to open up your air cap on the Paasche H, every paint brand and type is going to be different. Even the individual colors and sheens in a paint brand are going to be slightly different in how they lay down. So, you can't really use a "number of turns" method to adjust that. What I use at every painting session is a pad of sticky notes. I just shoot some paint onto that and adjust until I get the flow and coverage that I want before moving to the model.
With Testors enamels, I have tried both Testors thinner and lacquer thinner. What I have found is that I like the Testors thinner better. The end result just looks better to me, and you don't have to worry about any long-term adverse chemical reactions if you return the unused, thinned paint to the jar. When using their thinner, you do have to allow more drying/curing time. With Model Master gloss paint, I give it a minimum of 7 days before handling it at all. On a primed surface, shooting at 20 PSI (or 10 PSI for shooting the insides of curved parts), my first pass over the entire model is a mist coat that leaves lots of primer showing through. With each pass I get progressively closer and slower on my passes. The key is to slowly build up the color, so it isn't pooling, running, or sagging...and most of all, DON'T HOVER, KEEP MOVING. I haven't found that I have to allow any "drying time", as the previous pass will have already tacked up sufficiently to accept more paint on the next pass.
Just practice on spoons, and try to take YouTube videos with a grain of salt...it all comes down to what works best for you and produces the results that you're happy with.