Don't have DCS presently installed, but it's easy to find out if the F-5 has the correct basic drag.
With both engines shut down and at 13300lbs, flaps up and at 250kias, the ROD should be ~3750ft/min, with pylons and/or wingtip missiles or without.
With both engines at idle, DI 0 and at 270kts the ROD should be approximately the same as above but since too high basic drag can be conceiled by increased idle thrust, the idle descent result is naturally less meaningful.