all i can find on the internet are the attractive and repulsive forces between 2 magnets... where can i find the relationship between distance and attractive force , between a magnet and i.e. a piece of iron?
It is complicated to determine the attraction between a magnet and a piece of iron (or other ferromagnetic material). What you can do, if you know exactly how the magnetic field is configured, is to calculate the energy stored in that field and then calculate how much that energy would change when you place a ferromagnetic material around it. This is totally analogous to the problem Feynman analyzed on the Feynman Lectures, volume 2, chapter 10, section 10-5, with the difference Feynman did the math for electric fields and dielectrics. But the reasoning is precisely the same.
Anyways, unless you have a very simple system (such as a uniform magnetic field and a ferromagnetic plate), the math you will need to solve your problem can be quite messy.