Someone who is practicing all the kata through Sanseiloue, (probably Yondan and up)
should try to do without Master Uechi's "training wheels" for Wa-uke practice. (the basic training he taught of posting the upper hand/arm against the arm that descibes the full circle)
That was his idea (and an excellent drill) to keep the Wa-uke circle coming principally from the elbow not the shoulder and to restrict the diameter and thereby increase the speed of the circle.
What that drill produces is a small circle/oval , (performed by the elbow), within a larger circle/oval (performed by the hand and forearm ) all the while minimizing the exposure of the rib cage in terms of duration and size of the opening.
After ten years or so of practice, Master Takara's opinion is at Yondan, we should have the ability to perform the block correctly without needing the upper hand /arm to restrict the circular motion of the lower arm.
At that point, the wa-uke in all kata, including Sanchin, is seen as a closed circle, beginning and ending at the same point. I relate it to the numerals on a clock face. Looking at a student front on, the right Wa-uke starts and ends at 9 o'clock (counterclockwise) and the left at 3 o'clock (clockwise).
In Seichin, Kanshin and Sanseiloue, the blocks start at the same point as in the full Wa-uke, but, the circle terminates at roughly the half circle point as the opponents kick starts to be lifted vertically and you move your elbow out of Sanchin to keep his toes from reaching your chest.
I hope this is understandable! If not, let's talk some more.