Greetings from a rather far away place and time!
I just thought I could add some input into your conundrum of left handed writing. On this forum, I tend to lurk, but I occasionally pop in to see penmanship news and sorts to help people.
I don't know how committed you are when it comes to systematic mastery of Spencerian (I don't like calling it that, I prefer calling it ornamental penmanship), but what I can tell you that is if you want to start from the ground up life I did, you should practice arm movement.
For most people of the 20th century, they often learned cursive, or business writing was what it was called back then. Masters of the past often had a perfect cursive form, and thus, their skill with using the pointed pen rested in the arm. I cannot emphasize more this concept of arm movement. It has been nearly a year since I have gotten an interest in penmanship, but I have not seen a more greater improvement when I started using arm movement.
In other words, you will need to kill your fingers absolutely in terms of movement. Your script may suffer at first, but the effect is evident when you become better at it. For a shaded script like this, what matters is the grace and freedom of your lines, and in contrast, the thick shades that will accompany your lines. This is where the beauty in this script lies: the contrast.
My setup may differ from other left handers, but it is essentially what is recommended in "The Zaner Method of Arm Movement, Manual 144". My setup is at a slant; my paper is slanted, and my left hand is slanted along with it. Right handers slant counterclockwise, I slant clockwise. In this way, I don't need to use an oblique holder, but some, like M.P. DeCollibus prefer it so. Its kinda up to personal preference, but when it comes to movement, arm movement never fails. When you use your arm, your ovals will look rounder, and your forms more graceful. But still, some left handers will delineate and choose otherwise.
Anyways, my setup is in the attachments. One of the pictures shows my setup as it is rotated, and the other shows why I feel so strongly when it comes to using the arm. I have a really, really round flat oval. The paper is Drawing Paper Medium Weight from target, the ink is Higgins Eternal diluted down to 40% with water, the holder from Brian smith, and the nib is the Principal EF.
I hope this really helps.