It's a very difficult problem to solve. There are simple solutions only for a few very simple geometries.
The best analytical approach is through work and energy, force = d(energy)/d(distance).
As you pull the iron away from the field, you are doing work, which is equal to the force you apply multiplied by the distance traveled. This energy you supply gets stored in the new magnetic field, and depending on how you drive the coil, some may be delivered to the external circuit (how a generator works).
Among the many unknowns are what the field is at all points, and how the field changes when the armature moves.
The simplest geometry to solve is for the force required to open a closed magnetic circuit, for instance the electric door locks you often see on access control doors, as we know accurately the area of the poles, and the flux in them, and we neglect flux in the air. The force can be calculated as a 'pressure' times the pole area, the pressure proportional to flux squared. IIRC, 1Tesla gives 4 bar pressure, I may be wrong about the 4, but I'm not orders of magnitude wrong (do comment with the correct figure if you know or can be bothered to calculate it). You can work this out from first principles using the work approach above, using the energy stored in the newly created airgap.
You'll need to get out your 3D integration calculus or fire up a FEMM solver for any more complicated geometry. It's probably easiest to simply build and measure. The force will be proportional to current squared, until you hit saturation. Yes, saturation is a further complication!