I've had good success with a sander. I like a using #60 or #80 paper in my ROS. I find that my belt sander is a bit too aggressive. Once the curve is there, I clean it up with #220 paper. A sharp plane will work well, too, if you're good with one of those.

5/8" is about the same as a nickel. That's what I use as my standard for filleting tools for exactly the same reason - to make the cloth lie flat with no bubbles.

Have fun, you're almost there,

Laszlo

   Nickel, even a quarter, or the end of your thumb are good guides for the right radius. 

I prefer a longboard for cutting what will be the most important line on the boat -a long flexible-but-stiff board with some scrap wood handles glued to the back and a cut open sanding belt spray mounted to the front.

A hard rubber sanding block also works. After planing away the bulk of the material in the corner, use long, diagonal or spiral strokes along the corner, working from end to end and back again. The corner should have a shallow-angle "cross hatch" pattern on  it after a few passes. Some areas need more sanding than others, but use the same long diagonal strokes to blend them into the overall fair sweeping curve. The reflection off this soon to be shiny curve pretty much defines the character of the boat- it's worth getting right.