The "correct" spring length will depend upon the spring's rate (cf Hooke's Law), i.e. upon wire's material, wire diameter, coil diameter and/or number of coils. If any of these differ from USGI-spec, so will the force required to produce any given deformation. If Fulton's springs may require less preload to exert the same starting force as USGI, then they might reasonably be shorter. I don't know anything about Fulton's springs though.