Edit dimensions of Generic models in revit

Hi everyone, I am new here. I have been messing around with dynamo a little but not good enough to get me through this one. Any help would be really appreciated.

Currently I am working on a model where it has hundreds of “louvers” based on a generic model. The width of those louvers vary some are 1200mm some are 800mm etc. I wish to reduce all of them by 120mm so that the 1200mm would be 1080 and 800mm would be 680mm instead of doing them manually.

The problem the generic models are instances and cannot be edited via family edits.
I imagine the process should be: Selection of generic models>filter parameters>edit parameters by category.

Thank you for any help.

Are the parameters of the Louvers families that you want to edit Instance parameters of Type Parameters?

Okay then it is quite easy. It is important that you know you need to edit the parameters inside the project and not the family editor.

The steps are as following:

  1. Collect all FamilyInstances in the project which are connected to the Louvers Family
  2. Get the Initial Width Parameter (1200 and 800)
  3. Substract the reduction from the 1200 and 800 (1080 and 680)
  4. Set the Parameters back to the Family Instances

Step 1 is the hardest I think, because there are several ways to do this.
I always select 1 instance in the project and work from there. See picture below for every step

Really appreciate the super rapid reply and script!
Definitely much quicker and easier to use selected elements from model technique rather than filtering it through dynamo itself. Right now I managed to remake the script, however I am not certain about the “Number | Reduction” node. Is it just a normal number node? Or am I missing an package for that one?

Thanks again for your help.

Number node. but a code block or a number slider is also fine. I rather not use packages for such simple things