Your lastn is on the right track, but you're probably causing too much overhead by reading a byte at a time. Consider something like this (python-y pseudocode; I had a real implementation once at $job[-2] but I don't remember what I did with it):

BLOCKSIZE = 1024 (or so)
lines = 0 ; lines_wanted = 10
seek to end of file

while lines < lines_wanted
  seek back BLOCKSIZE bytes
  read block into buffer
  pos = end of buffer

  while pos > 0
    use rindex to find \n starting at pos
    if found
      lines++
      pos = index of \n
    else
      last
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Update: You of course need error checking on seeks to make sure that you don't try and go back past the beginning of the file. And you would want to remember where the buffer starts so that you can use that (plus pos) to determine where to seek to when you start reading forwards.