Dynamically update form values in a FormArray using valueChanges

I know that we can use valueChanges on a FormArray and view the changes as it is being typed. My question is straight forward: how do I perform the array.at(i).patchValue in valueChanges? I would need to know the index of the row where I am keying the value and update FormControls. What I would like to achieve is for e.g. in a calculator, I want to dynamically show the value when I add two numbers without having to click outside of the input field.

ngOnInit() {
  this.form = this.fb.group({
    fdnUnitPrice: [''],
    cap_values: this.fb.array([this.fb.group({
      name: '',
      fdnTotalShare: '', 
      fdnVal: '',
    }
    )])
  })
  this.onValChanges();
}

onValChanges(): void {
  this.capValues.valueChanges.subscribe(val => {
    console.log(val.length)
  })
}

In my code above, what should be the result is in fdnVal, it should show the value when I multiply fdnUnitPrice with fdnTotalShare. The next row will probably have a different value for fdnTotalShare, so fdnVal should be different.

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