Sometimes you don't know the exact line you want to go to in a file, but you can infer an approximate percentage. I was browsing a 200 meg logfile with Vim yesterday and guestimated the area of interest was about 10% back from where I was at (85%). You can easily jump to a desired percentage in a file by typing in the percentage followed by a percent symbol.
Jump to 50% of the file:
Easy to remember to say the least ;-)