Hello
@Simone Lettering There's a good reason for this, but the first thing to say, as ever, is if something works for you it can't be wrong, can it? This area of posture and ergonomics is open to a lot of personal variation, and so long as you aren't giving yourself back ache or carpal tunnel syndrome you should trust your instincts.
With that out of the way, the business of flat table for pointed pen and slanted board for edged is largely to do with ink flow rather than anything else. Because of the way a pointed nib works and the way ink will cling to it, some help from gravity is necessary. Some people
will use a tilted board (which can be more comfortable), but it tends to be a matter of a couple of degrees or so. If you decide to use a quill for copperplate or Spencerian, you might find that a steeper angle is better because ink flows more readily from a feather than from steel. The typical set up for edged pen is a board tilted to 45° or more - this is to slow down the flow and prevent pooling. In this case the pen is not far off horizontal.
I usually write on a flat table, but most of the Spencerian writers I've met (all of whom are way better than me) favour a board on the table with a thin lath across the back edge to give a very slightly angled surface. You are right to identify the relationship between chair and table as being very important, but a gently tilted surface can give you a bit of extra leeway.