Dave --

I wouldn't scribe anything.  I've finished both of mine just as you plan to, and used blue painter's masking tape, period.  First, I taped off the edge that was not to be varnished and did all the varnising.  Then I pulled that tape off, and taped off the varinished deck, and primed and painted the base color.  Then I pulled that tape off, and taped off the portion of the base color I wanted to keep, and painted on the second color.

It's easier than it sounds.  If you use a narrow tape, you'll find that you can curve it pretty easily and smoothly just by eye.  I carried the paint up 3/4" on to the deck to hide the fastners (I used screws instead of nails) and to attractively frame the varnished deck.  Then I ran curving lines of tape across the deck just forward and aft of the cockpit; one of the curves followed a varying radius curve that approximated 30".  Once the first layer of tape is down, I backed it up with an addition line of tape, and then used cheapo masking tape on top of the second to attach newspaper to furhter mask the protected surfaces. It all worked pretty smoothly.

One additional thought.  I wanted a tightly curved joint between the lines running up along the side of the decks, so I joined the two strips of tape with a piece from which I had cut a curved mask, using a quarter as the patetern.  To my eye, it looks better than having the two lines come together at an acute angle. 

Two cautions: Be sure to rub the tape down firmly to ensure a good, tight seal.  And store your tape in a plastic baggie so the edges don't pick up dust, hairs, or other garbage that would interfere with creating a clean, sharp line.

I'd attach a photo if I could figure out how to glue a JPG file to this message; none of my pics are on the web, so they don't have URLs.    :-(