How the heck do you close a tail? :c

So I'm working on a tail for my first suit.

Here's the pattern for it. There's also a thin tapering panel that runs along the bottom, since I want to use a different color fur there.



It's somewhat wide at it's base, and I want it to have a flat base to rest against my lower back/rear.

I'm making a mockup tail out of muslin since I don't have much sewing or patterning experience. I have it all sewn up except the base. I just can't seem to get it to work out, no matter what I try. The panel always ends up too big or too small, or it gets all wrinkly and weird.

So my question is, when making a flat-based tail, how in the world do you figure out what size and shape the base panel needs to be to fit the hole properly? Is there an actual method for doing it, or is it more guess-and-check? I feel like I'm overcomplicating it. :c