Airbrush practice Medium

Depends on the type of plastic.  Large milk jugs are polyethylene, which is a bear to get paint to stick to.  I personally like the spoons.

I use sticky notes for various things while modeling, and checking spray patterns is one of them.  When one gets used up, I just pull it and have a fresh one ready to go.

To the OP, I have a Paasche H also, and I've tried many times to change to something "better", but I just keep coming back to my Paasche H.  You've definitely got a simple, reliable airbrush there.  You'll love it.  

For checking your spray pattern, or the size of lines, a simple piece of plain paper works just fine. Keep it simple.

if you are looking to practice your steady hand with the airbrush i reccomend using a small glass sheet to practice on.  You can paint on it and than scrape the paint off with a razorblade and do it again.  

If you are trying different techniques like thinning paints, using different PSI settings and that sort of thing i would reccomend using a paint mule, An old model that you dont care about. 

The best thing to practice on is an old model. An old build from once upon a time makes a super paint test mule. 
You can learn to contend with all the challenges of airbrushing a model: shapes, angles, overspray, masking, etc.