Topband: What percentage of the length ...

[W0UN -- John Brosnahan]

BTW   10% plus two feet is the recommendation of the
Lineman's and Cableman's Handbook, 7th edition, by Edwin
B. Kurtz and Thomas M. Shoemaker, page 11-3, both in the
text and in Table 11-1.

Sorry I didn't send you the reference.  I did to someone who
questioned me about it off-reflector.

Of course it is soil dependent and in rock the depth is significantly
less.

--John  W0UN