You should be able to accomplish what you want simply by triggering a 'change' event on the inputs of type "checkbox". 

- Form -

All Controls: 

Control (1) : Yes/No: 

Control (2): Choice:

Control (3): Calculated Value:

Form Variable: 

Rules: 

Formatting for the Choice Controls: 

Custom Javascript: 

Copy  / Paste Code: 

NWF$("[data-controlname='checkAll_Toggle'] input[type='checkbox']").on("change", function(event){
  if (NWF$(event.target || event.srcElement).prop("checked")) {
    NWF$("[data-controlname='someChoices'] input[type='checkbox']").prop("checked", true).trigger("change");
  }
});‍‍‍‍‍

How does it work? 

It puts a "change" event handler on the 'Toggle All' Control. If you click on the toggle all control, it'll invoke the event handler. 

Once the event is being handled by our code, we check to see if the Toggle All is being set to On or Off (true or false). 

If it's true, then we get ALL of the inputs of type "checkbox" that are children to the "someChoices" control, and set them all to checked, and then trigger the change event on them, resulting in the form variable being updated, and the 'length' of the array being pushed to our Calculated Value Control immediately. 

So.... 

On Load: 

Two Items Checked: 

Check All Checked: 

I hope that this helps you accomplish what it is you're trying to do. 

I'm glad that it's going to help, and this is indeed a wonderful community!