No magic numbers, sorry 'bout that. Try hanging a 40 meter dipole
from the same feed point as the 80 meter, fed with the same feed
line. Tune/trim for use near the 40 meter frequency you want, then
use that tuner. The same thing will work for any other band you want
to add to it, and will certainly be better than a 'G5RV'.
- 'Doc