There is no such option in standard.
The tolerances are defined in the purchasing value key. and the purchasing value key is entered in a material master. There is no relation to a quantity at that point of time.
From here it will default to the purchase order after you entered the material number and pressed enter. So even this moment where the user hits enter can be before the quantity was entered.
You may need to develop your logic in a BAdI, not sure if you can change the value of that field automatically while saving or if you only check if the user has entered the correct value and send error messages to the user if this is not the case.