That is an odd reply from them. Usually, if they have a question along those lines, they will simply require a lab sample.
PPM of licorice root doesn't matter per se, it is the PPM of the limited chemical Glycyrrhizin. As Glycyrrhizin, it is limited to 0.1%, or 1000 PPM. Usually, if you indicate a limited amount of licorice added to the spirit in the process, it guarantees that you can not reach that limit. Likely your formula is in the gray area where the upper bound of your formula would not keep you below 0.1%? Because 1 gallon of spirit is 2987 grams, and if all 24 grams licorice was introduced into the spirit as Glycyrrhizin, then you would be 0.8%, which is well above the 0.1% limit. So, they need some way for you to demonstrate how you will ensure less than 0.1% using the quantities and method you describe. One way is if you knew the weight of the licorice before and after distillation, worse case assume all that is Glycyrrhizin, and calculate result of upper bound. It should be below 0.1%, IMO. Or offer to have them test it, they don't charge you.