Electromagnet push/pull force formula

I remember seeing a forumla that calculated the seperation force in grams of an electromagnet using the following factors, power (VA), core cross section area in square milimeters, wire guage and length. The formula allowed a "close enought" force calculation and could be solved for other factors as long as you provided four of the five variables. My need is to calculate how many feet of 30 gauge magnetic wire wrapped on a 3 sq mm iron rod and engergised by a 2A, 18V DC power supply to counter act a permenant magnet the has an attractive breakaway force of 4 ounces. The application is to move a model railroad switch point set side to side changing the track path. The thought is that a permenant magnet attached to each end of the tie bar with common pole pointing toward one another and a latching plate at the ends of the tie bar range could be moved back and forth by energizing an electromagnet mounted between the 2 permenant magnets. The electromagnet would repel the closest permenant magnet while attract the more distant one. The range of motion is about 5mm to and fro.