I haven't used their rennet, and each batch has depending on storage conditions and age, different strength, plus the milk you are working is different from others.
Normally the manufacturer's guideline dosage is the best place to start and then adjust up or down based on your experience for your milk (which changes over season) and your rennet (age and storage conditions) and your cheese you are making (ie some recipes call for pre-ripening before renneting which lowers the pH and makes coagulation easier).
Three different methods for determining when to cut a rennet coagulated curd are listed here
, if your recipe calls for cutting the curd at more or less time than when the method you follow shows ready, then you need to adjust your rennet dosage up/down on the next make to closer match your recipe.
Therefore, if you are using the same milk and same rennet and again making a Parmesan then go with your previous dosage and adjust if that curd was too fast or too slow.