Flooring Tile - grout width

I need to determine the grout width for some "tiles" which I am going to paint on my stairs. (You can read about the plan in the painting forum listed under "painting stairs".)

http://forum.doityourself.com/showthread.php?t=244121

After doing some drawings, I determined that I will paint 8 inch "tiles" on my stairs (other choices were 9" and 12"). I am also going to paint the "tiles" which run over the end of the stairs like you would find on a counter, such that there will be a "grout" line about 1-3 inches from the edge of each step.
I am going to paint the "grout" first, so it will be lower than the "tile", so I am going to use masking tape to keep the grout line. The problem is that I have found only 1/4 " and 1/2" fine masking tape for my grout line. I want to make this look as natural as possible (although I'm not trying to fool anyone). Which width of "grout" line would look best? If the answer is neither, what would the correct width be if I could find the tape width, or was willing to cut some tape narrower? thank you for your knowledgeable opinions. I plan to start in early January.

3/16", and you plan on walking on these painted stairs, without it wearing out real fast?:)

I plan on putting 2 or 3 coats of polyurathane over the paint when I have finished.

BTW I just returned from Universal Studios Florida and the Shrek exhibit had hand painted blocks of "stone" on the walls outside with hand painted "grout". I've photographed the walls for further study. They did not use tape to make grout lines but it was hand painted. Perhaps what I need is the correct size artist's paintbrush for the grout.

and a very steady hand :blah:

Good Luck and post pics when your done...you have me interested.