Scarecrow,
One important thing I'd say is don't necessarily try to imitate a picture of a thumb ring shooter, as the maximum draw of the bow won't change, but the draw length of the archer will. The Kaya has three lengths of bow: regular, janggung (extra long), and jangjanggung (extra, extra long). The first will probably draw back to about 33", the second an inch or two more, and the third to 36". It isn't stacking that you are feeling, but the end of the draw for a particular bow.
Also, when drawing, you should probably draw a little lower, near your shoulder; if your arrows are the correct length for your draw, you are at full draw when the back of the point touches your bow-hand thumb.
I hope that helps!
Thomas