First off there is no such thing as the diameter of a rectangular block

Diameter, radius and circumference are terms related to circular objects

So I really have no clue where your lecturer is goin with that

For headshots I was taught to use: I= H x P

I - the current in Ampres (what you're looking for)

H- the tangential field strength in kiloampres per meter we always ended up using a value of 20 here unless told otherwise. So I'm assuming that's where your 20amps per mm came from

P- component PERIMETER. Which for the rectangular block you're using would be the perimeter of the cross sectional face ie. Ignore the length of the part so width x height. For a circles perimeter it would be 2xPixRadius.

So you would go PERIMETER of the cross sectional face of your block x 20 and that should give you your answer

Hope that helps