Suspect that FL250 is chosen as the altitude where difference between IAS and equivalent airspeed becomes significant in this particular type. I.e., above FL250, the F40+70 knots IAS no longer provides sufficient margin above the stall as the EAS has become too slow. On the 146/RJ the minimum clean speed increases by 20 knots above FL270. As alluded to by Snakecharmer however, this figure is largely of academic interest as the poor thing is more often found stooging around at ~FL250