Different protection devices eg fuse thermal overload CB & other equipment like magnet contactors are selected as certain %age of FL current. What should be there %age with respect to FL I or rated I?

it depends upon your load specifications (phase, type of load, etc.):

in circuit breakers, instantaneous trip: 200-800% of FL current while inverse trip: 150-250% of FL current...
in thermal OL relays, 115-125% of nameplate current rating...
in fuse, for time-delay: 150-300% of FL current, whereas, non-time delay: 150-175% of FL current...

If you are looking for High Voltage fuse application, you must refer to IEEE C37 Series of standards and there are UL standards of applications for Low Voltage fuses. They deal with specific application for each equipment.