As Krusty has already pointed out after any loss of power (reduced power in a single or powerplant loss in a twin) you should find out how much performance you actually have. Set maximum continuous power, maintain altitude and if speed reduces to blue line (or best rate in a single) then allow the aircraft to descend and see what capabilities you have. This will be the best case scenario understanding that as gear, flap etc are added performance will suffer.
I use what i learn from this to ascertain the seriousness of the situation. If the aircraft can maintain safe altitude on the power available PAN call and land at nearest suitable (safe) aerodrome. If the aircraft can not maintain a safe altitude when the book says it should then an immediate MAYDAY is required with landing at the nearest available airport/clear area etc... With a piston like the PA31 there is additional management of the live engine to worry about with mixture and temperature control being critical if a longer flight on one engine is to be considered.
Never assume any multi-engined aircraft will maintain height or perform to book figures after a failure, jets and turbo-props included as history has proven time and again.