In my opinion there is no hard and fast rule, buts what is true is that :
• The heavier you are = the more chance of cracking.
• The more post is exposed = the more chance of cracking.
You just can't get away from that ...
Some makes / models / materials / designs are more prone to cracking than others - if yours is one of them, factor that into the above.
Frame ID and post OD (regardless of that the spec says or the post has stamped on it) have a tolerance - a poor fit will alter its fatigue resistance and should be factored in.
Even the bloke / machine that welded it could alter its fatigue resistance on 'identical' bikes made on the same day.
At the end of the day, no one on the net could tell you 100% if it will be ok or not - it's all guesswork that needs consideration / good judgement / good luck ...
Whatever option you go with, the only sure thing is running a post that long has increased the chance of cracks / failure.
Hope that helps