Okay.... that is what they call a Deep Vee in clothing sewing. It's mostly used on necklines. This may give you some tips.
https://www.youtube.com/watch?v=4R54KCmQMps
She is sewing a standard 5/8" seam used in home sewing for clothing. You would do a 1/4" seam. Hope this helps some. I watched it and it said use seam sealant. That makes a hard edge. I wouldn't do that. You won't be turning the piece all the way, but sewing an insert in. I'll look for another video.