The LD-50 listed for LSD on erowid is 12mg but I think this is highly conservative amount. I have heard other estimates of ~50mg. I am pretty sure if you take even 1mg of LSD per KG you are going to have a pretty substantial chance of death. The pharmaceutical index for LSD makes it extremely non-toxic, but I personally think you may very well be risking death at doses as low as 12mg, and I sure as fuck would never take a thumbprint.