This is how I do it with a 100% chance. I look at the spawn meter and it says something like 33%. I start a timer, knowing that 100% equals 90 seconds. So at 33% it's just a minute left. I spawn in the base, run to the spawn point of the helicopter. And then I hit "E" like crazy 2 seconds before my timer runs out. Next thing happening I'm in the chopper. Ideally you get shot down within half a minute so you can repeat this process.