I already did use the optional method
That tends to be the most generous with big FIP loads so you might be looking at a service upgrade. That is not unusual with these electric tank less water heaters. Chances are they would never have a problem and an analysis of the other loads might give you better guidance but we tend to design for what might happen, not what usually happens. Think Christmas eve, everything in the kitchen is going, "Griswald" lights in the yard, heat is blasting, someone is in the spa and they turn on the hot water.
