Phew. What a title for a post! But, basically, that's what it is.

To explain: I was looking for a way to make dynamic shelves, where I could not only determine the distance between the individual boards, but also the number of them. Simply put: I want a family where I can say: shelve A has three boards that are 40cm apart, while shelve B has four boards that are 45cm apart.

To get to grips with the technique I first made a trial family, using my trusted cubic meter:

Great fro trying out things.

I then set up a family using the Generic Model - Floor Based template, because my cubic meter sits on the floor. You should use the appropriate template when using this technique for other things.

Anyway. I loaded the cube into the family, and placed it at the center of the family. I then created a reference plane to the right and gave it a defined distance with a parameter calles "distance". (If this is too fast for you, please read the manual on dynamic families and labels.)

OK. Now comes the tricky part. I then went ahead and created an array from the cube, and selected the following settings:
I then made the array strecht from the central reference plane to the one I had set up.

So far so good. The next step is to create a parameter for the number of cubes in the array. Select the cube, and a temporary dimension will pop up the the number of items on it. This you can select and add a parameter.

Well. That's it for the basic setup. Now comes the "programming part":

Open the Family Types dialogue box and set it up in the following way:
Create a parameter called "block distance" (or whatever :) ) and group it under Dimensions. (This is not strictly neccessary, but helps with usability for others.) This is the distance between the cubes - more precisely the distance between the centers of the cubes.

I then let the distance be calculated by multiplying the number of cubes (minus one) with said distance.

And presto, it works!