pedestrian: i dont know if you can call it a formula .. but personally i look back to the last 3 rounds .. i look at how much the average increases between those rounds and continue from that .. or i simple do 1.5 times the last difference

its quite easy to copy that and add 1 ...
i think the 'formula' used by other players is a bit more complicated .. but as said .. the goal of this game is to guess the formula used by your fellow players and use it to your advantage