When the pit stalls are in the second half of the lap distance, it's considered behind the s/f line. When it's in the first half, you're ahead of the s/f line...
It's not that complicated. You start on Lap 0 (whether on the grid or from a pit stall). When you cross the finish line for the first time, you're on Lap 1.
Daytona Road Course (67) is a good illustration of this, with the start line on the back straight and the finish in front of the grandstands. If you start the race from the grid, cross the "start" line, but don't make it around to cross the finish line at least once, you'll be classified DNS (did not start) ("start" is in quotes there because it's meaningless to GPL).
If you miss the start at Daytona, and join from pitlane (stalls are behind the finish line), you join in time to make it to T1 in front of half the field (test done with 67F1 cars). So that's one track where shenanigans are possible; I'd assume there are more (but they don't come to mind atm).
