I think I would use a compass to make a stencil on a sheet of clear plastic (like overhead projector paper), cut it out and use that to stencil in the curved corners. That would allow you to measure and get eah one consistent. Try it a few times on regular printer paper until you get the dimension/curve you want, then transfer the final product to the plastic sheet, so you can reuse that over and over on your walls.

I shared your post with a few of my blogger friends who might know of someone. Hang tight.

I don't know of a tutorial, but like Leslie suggested - if you're taping off the straight lines, how about making a stencil by using poster board to make a rounded (elbow-noodle shaped) the same width? Maybe lightly trace it with a pencil and free-hand paint the rounded corners? (still tape a perfect rectangle to start and fit the ends of your rounded stencil in the corner against your tape line...hard to explain). Or change to straight lines like the French panels shown at http://www.cuttingedgestencils.com/stencil-f....

I can't quite make out the picture, however if the curves are just paint, I'd suggest have a local signmaker create corner decals to be used as stencils. Stick them on, then paint, then remove decal. Instant stencil! Then add your extended lines with painter's tape.

Another hint. When you get everything taped off, "seal" the edges of the tape with your current wall color where you will be painting. Once that dries, then paint the white or accent color you will be using for the lines. It will make your lines so much sharper because it prevents the new color from bleeding under the tape.

You are all so talented, that's all I have to say :)