I want to Calculate the Static Pressure the AHU should be Designed at, if the Air Velocity (FPM), Air Flow rate (CFM) and the Duct Length is given. Is there any direct Formula for that or a series of relations? Help needed

It's a little more involved that a simple formula. Look up "Manual D" which is the manual for duct design.

Lengths, cross sectional area, type of pipe (frictional losees), number of fittings, kind of fittings, transitions all contribute. So does the type of grill. Your looking to be in range of the static pressure permissible by the furnace and the CFM is's capable of delivering at that static pressure.