I’m making a beam from a 2x8, should it have a rounded top all away across, or should there be some flat left in the center?

What is the shape of your fleshing knife?

Just round off the edges. And the shape of your knife has nothing to do with the shape of your beam. Your knife is just about flat to the surface of the beam if you're using it right.

It’s up to you, find what works best. Unlike the beav I like my board to match the curve of my knife for at least the the top portion. When making my favorite board, I would sprinkle baby powder on it and then push my knife down to see if there were any low spots.

just make sure it smooth and no knots where you will be using it

The more round the beam the more narrow a piece of flesh you will remove. Especially beaver. My beam is rounded but I've gotten use to using it. Scraping coon works well because my knife is closer to the curve of the beam and goes much quicker.