Please take a look at the attached sample file for a solution to this issue. Here's how I've solved it.
1 - It's important to add Transform
operators for adjusting the Y Size of segments if you are going to drive the values with more complex equations. This is because segment's values can not always be edited directly using an equation or values exported from another segment. In most cases you will get a warning about this but it seems not in this case.
2 - To calculate the correct size for the segments you need to work out the height of the door minus the size of the top and bottom frame and any glazing bars. The number of glazing bars will be one fewer than the number of glass segments.
3 - The height after these values have been subtracted should then be divided by the number of door division. To write this full equation out in english would look something like this...
(Door Height - Top Frame Y Size - Bottom Frame Y Size - ((Door Divisions - 1)*Glazing Bar Y Size)) / Door Divisions
Expressed as nodes it looks like this:
You could also write it as an expression, but I've avoided that in this case so as not to overcomplicate things.
I hope that helps, please let me know if you have any further questions.