@pmolan, great question. This method does work for calculating the precip rate – equation found here. Variables A & B would become one – note that you’ll need to convert your meter reading to GPM (gallons per minute).
Catch cups allow you to find the efficiency of which the water is being applied to your lawn. Without catch cups, this variable is unknown – yet plays a key role in calculating your watering duration. For more details, please refer to this support article, step #9 and this support article, see Efficiency (DU).
Let us know if this addresses your questions.