The answer to the original question is dependent on input not
specified by the original questioner, i.e. which continuity?
In TV series one, the answer is exactly 1/4. (Nobuyuki
is not descended from Youshou, who hasn't been on Earth
long enough, and Youshou is pure Juraian.)
In OAV, the answer is complicated. Youshou himself is
only 1/2 Juraian, but Nobuyuki is descended from him (and
Tenchi's maternal grandmother may also have been, for all
we know. For that matter, Funaho may have been descended
from previous visitors.).
In Shin Tenchi, the answer is zero. Ditto Pretty Sammy.
themselves can't! Yosho, from what I've seen in the OVA, have
a similar shaped force shield he used to defend against
Kagato. Not sure abt the nature of that. Washu probably
figure it out... heh
I believe that the '100 Tenchi Secrets' document said that
Youshou's LHW (used against Kagato) could only be generated
by 'normal' Juraians while holding a first, second, or third
generation Master Key.