Uh oh.. the age ol question is here again, and usually I run away at this point. Because, like Erica.. I didn't give a hoot about positions, axis, parallel, this or that.. it all confused me too much, so I just sat with the oblique pen, tried to get comfortable and just started writing. I am actually kind of hesitant to explain how I do it, because I think I am not holding it the 'proper' way. But.. for me the ends justify the means! Some master penmen have weird grips and some hold in a funny way, too. They say to sit up straight, but I've seen some masters hunch all the way down (due to bad eyes I suppose, I do too). And some poor left handers practically are inventing space technology to write with an oblique pen. Don't let all these weird mathematical arm measuring stuff detract you from the actual art.. read their advice and see what works and what doesn't.
This is how I am holding my oblique pen for right handers...
First just sit comfortably in front of the table as you do naturally. Now put your right arm down as if you're writing naturally.. hold the oblique pen as if it is a regular pen. Your wrist should be relaxed, not twisted weirdly. If your whole arm is on the table, okay, if just the forearm is on the table, that works too. Just be comfortable and make sure your arm and wrist can move around freely.
Then with your left hand, twist the paper anti-clockwise so it is parallel to your right arm. Now you will notice that in this way, the oblique flange should be making the slant for you naturally on the paper. If not, then twist the paper some more. Above all, your wrist and arm should be unrestrained and comfortable, let the flange do the work for you. Don't worry about the angle of the pen staff relative to your stomach or whatever, we're not engineers who cares omg. If you gotta twist your body a little, okay, if you gotta twist the paper, go ahead! As long as it looks kinda sorta like what everyone else is doing (and you're not in discomfort), and you find you can make amazing letters in this position, then stick with it! Just be comfortable, sit up straight (or as much as your eyes allow) and arms and wrist is unrestrained.
After some practice and familiarity you'll notice the stuff everyone else is talking about.. some lift their forearms for flourishing, some twist their wrists for shades.. you'll come across these in time and realize it makes sense. But don't worry about it yet.. the little epiphanies will come in good time. Just do what you can to get started.. it's okay to readjust the position in the future once you find you have to.. but right now do what you have to to make the pesky oblique holder your best friend!
Maybe after a few weeks of practice you can take a photo for us and we will critique your sitting or work.. remember it's a learning process, no one ever gets it right the first time!