ngx-formly autofill one of three fields if two are already defined

i built three fields with formly. Together they can have a max of 100%.
What ist best way to autofill one of the fields, if two have a value.
For Example:

First field: 30
Second field: 20
Then the third field should automatically filled with: 50

What ist the “best” solution for this?

I already found a almost working way but iam sure my solution is not the goto. I used the formly lifecycle hooks to get and send the current value of my three fields to a service and calculate the value for my last field in there. The problem was that it generated a lot of traffic and slowed down my application. If i tried to write my own number into the last field, the input got instantly overwrote by the value which the service returned. I want the user to be able to overwrite the autofilled field and autoclear the other fields

My approach:

The hook in my third formly field which checks if two fields are filled:

        hooks: {
            doCheck: (field: FormlyFieldConfig) => {
                let fields = field.form.getRawValue();

                if ( 
                    fields.fieldOne.value !== null && 
                    fields.fieldTwo.value !== null &&
                    fields.fieldThree.value  !== calculateFieldsService.calculateFieldThree(
                        fields.fieldOne,
                        fields.fieldTwo
                    )
                ) {
                    fields.fieldThree.value = calculateFieldsService.calculateFieldThree(
                        fields.fieldOne,
                        fields.fieldTwo
                    )   
                    field.form.setValue({...fields, ...fields} );
                }
            }
        }

The service which should calculate my field value if two fields are filled:

@Injectable({
  providedIn: 'root'
})
export class CalculateFieldsService { 

  calculateFieldThree(fieldOne, fieldTwo): number {
    if (fieldOne.value >= 0 && fieldTwo.value >= 0) {
      let fieldValue = 100 - (fieldOne.value + fieldTwo.value);    
      return fieldValue < 0 ? 0 : fieldValue;
    }
    return null;
  }
}

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