Tossed the vertical stab down on the bench and scribbled a rough outline for the rudder. Used some sticks to make a rudder out of 5/16 x 3/8 balsa.

The counter balance is 1.5" tall. From the LE of the counter balance to the TE is 6 inches.

Bottom of the rudder is 6 inches from LE to TE.

Basically, the taper is equal to the distance on the top of the vertical stabilizer. About 2.5 inches.

Will it work? Probably

Did I use some scientific method of figuring out how many square inches to make the rudder or how much taper on the TE? No

I used an old redneck engineering principal known as, "That looks about right."