Upon request from other users i've create a tutorial on using nested families to create parametric arrays.
Difficulty: Intermediate
Let me know if its helpful...
Cheers!
Note: As beegee mentioned the final formula for the no of louvres should read
Heigth /(Free Air + Louvre Height) and not width as shown. - THIS HAS NOW BEEN AMENDED
Please note this has now been updated and is available from my blog