How to Design a Solenoid for an Electric Door Lock Releaser?

[I_am_learning]

I am trying to make a an electric door lock releaser. I am going to use a solenoid to make an electromagnet that will pull down a handle to release the lock. The electromagnet has to act from a distance of 1 cm. And it has to impart a force of at least 10 N. The problem is to design the electromagnet to operate at 12v. I mean to ask, how many turns of wire?, what diameter of wire and the iron-core?. Whats the method to find them?

 

[I_am_learning]

Put it in short my question is--
, Given a solenoid, you can find the magnetic field strength by B = uNI.
But How do I calculate how much force this field will impart in a piece of iron some distance away?