Thanks, guys - I've read all your responses, thought about them and
have been Googling to learn more. It looks to me like Bob W's "rant"
was correct, and that JJ's method is the best way to go.

Brian said "there is a known range of allowable CofG's in terms of
%MAC which is similar across a wide range of airframes", so I decided
to see what that might be. The best I could tell a reasonable range
was 15% to 35% of MAC. However, it's really a loosey, goosey "range" -
the HP-18 Bob K referenced has a 25% to 40% of MAC actual range, while
a DC8 has a 8% to 18% of MAC actual range! I did discover that by
using the 15% to 35% of MAC range Frank Irving's optimal CG is 75% to
100% of the allowed CG range (where 0% is the fwd limit and 100% is
the aft limit). It isn't very precise, though, and it doesn't agree
with the Akaflieg Braunschweig findings.

I then calculated the arithmetic mean chord of the LS8 wing by
dividing span by aspect ratio, and got 700mm. This looks about right,
since the root chord is 900mm, and yes, I know it's not the MAC. Then,
I found a scale drawing of the LS8 at: http://www.dg-flugzeugbau.de/Data/3s-ls8-s.pdf
If I'm reading it right, the 25% MAC will be located 225mm behind the
wing LE. Using the arithmetic mean chord of 700mm and the allowed CG
range of 280mm to 400mm behind the wing LE, I calculated that the LS8
has a CG range on the order of 33% to 50% of MAC. That seems wrong
enough that it isn't worth the bother of actually going through the
geometric excercise of calculating the MAC on the triple tapered LS8
wing. I cheerfully admit there's a possibility that I don't know what
I'm doing, but at this point I'm going to drop Irving's approach.

This gets me to JJs advice. I'll follow the 2001 Akaflieg Braunschweig
method, and use the tail tank to set the CG to 65% of the allowable
range. Once I'm familiar with how she flies there, I'll move it back 5
to 10% at a time until I either reach 90% or get to JJ's criteria
point (I have to trim forward when entering a thermal), whereupon I'll
bring it forward 5%.

I'd be grateful for any further comments or suggestions.

-John