Eva Haberman, the actress who played Zev at the time she was transformed into a loveslave, and got all mingly with a cluster lizard, is about 5'6" tall. I would guesstimate that she would weigh about 125 lbs. Now, the head of the cluster lizard that got smooshed in the machine with her was about the size of a human head, which on average weighs about 8 lbs. Lets add a bit for the "alien" factor and say 10 lbs.
Therefore: 10/125 *100 = 8%
Note 1: This is of course assuming that the proportion of cluster lizard remained equivalent when Zev was transformed into Xev, played by the lovely Xenia Seeburg, and my avatar
Note 2: If you have no idea what I am talking about:
a) Watch Lexx or
b) be comforted by the fact that neither do I, mostly
kazza, edonkey.. to name a few matey, i never watch shows on TV, too out of order i just download an entire series by number and watch it when i got time.