I'd agree with 1/4 inch of overlap. You can either do it on a mirror as suggested above or if the spar location is suitable, cover the rear half of the wing to the spar and trim the cover flush with the leading edge of the spar and then use the 1/4 inch or so of the spar or spar web as the over lap to cover the front portion of the wing. If you built the wing right, this guarentees a straight line and is much easier than tacking the pieces together and transferring them to the wing. If the leading edge is sheeted, rather than open framed, its's even better as you can use the last 1/4 of the sheeting or spar for the overlap.

Using the spar as the overlap works well as it allows both pieces to be firmly anchored and sealed. Also the resulting 1/3 - 2/3 distribution of the colors is usually more appealing to the eye than a 1/2-1/2 split given the rule of thirds etc. You can also use a 1/8 inch trim stripe set back 1/4 inch from the seam. This can be the same color set into the other color or a third complimenting color. This tends to break the hard line of the two contrasting colors and again is more pleasing to the eye. It's adds a bit of depth to the seam and also will distract the eye from any minor imperfections in the seam which would otherwise be very glaring. Using a 1/8 stripe 1/4 into the forward color also provides the same 1/3-2/3 distribution in small scale and compliments the main color distribution of the wing very well.

Another easy to do trim thing is to use the leading edge color on the center section, which on most planes is also sheeted and therefore easy to cover in the same fashion as a covered leading edge. This leaves a rather hard 90 degree corner but this can easily be rounded. Make a round or oval template and then cut a piece of film to fit the corner. On my last aircraft I used an oval window from the little photo albums that come with your pictures from Walmart. I just marked the edge of the oval in the two spots where it ran tangent to each edge of the cover and used this as the template to cut the curved edge of the monokote. (Leave a 1/4 in front of and to the inside of the trim piece to get the required overlap on the leading edge and cente section). You then flip the template over to cut the mirror image piece for the other side. The oval shaped curve used lenghtwise with the span provides a very smooth continuous curve that compliments the wing. The trim stripe then follows this curve around and off the trailing edge. The only care required on the small trim piece is to seal from the corner outward to avoid trapping any air. Als when covering the aft section of the wing, only overlap a quarter inch or so onto the center section to avoid a large area in which air could be trapped when coverng the center section with the other color.

In any case, use a straight edge and a very sharp (new) exacto blade when cutting the piece(s) that will lay on top . Also don't get carried away with too much heat when sealing the final seam and run the iron parrallel to the seam not across it as this will tend to prevent you from pulling any color from one piece onto the other.

I like monokote and prefer it to most other films, but if you are covering a foam wing, I'd use econokote due to the lower heat required. It also has just a little more stretch, I think, over solid or sheeted surfaces and works better on tight compound curves on fuselages. The thing with either film is to work slow and keep pulling the film as you work around the curves and again get a 1/8 to 1/4 overlap on the seams between the pieces.