Timing is definitely odd, now I look at it. Khamul's works whenever his unit takes a hit, and Rampage goes on until either all rolls miss or the formation is all killed.
So, despite what I initially thought, it does seem logical (without a specific FAQ to handle the timing) that Khamul's saves will save hits, but the rampage will continue on and apply new hits, until they either fail the to-hit roll (which could be 1+) or Khamul and his formation die.
On the other hand, it also seems plausible that you do the rampage, accumulate the total hits (either up to all rolls failing or the total number of models in the formation) then apply Khamul afterward.
Sorry, that is probably no help at all. Except insofar that now I have posted in relation to a rules question, someone else will be along shortly with some small reference I have missed
