What that does, and what it does not do
Hard water is a mineral load, and the minerals come out of solution on the hottest surface they touch. In a storage tank that surface is a large element or a large flue, the deposit builds slowly, and flushing manages it. In a tankless unit the same minerals land on a small heat exchanger that the whole flow passes through, and the deposit narrows the passage it is sitting in. Same water, same chemistry, and a much shorter interval before somebody has to do something about it.
So the honest reading of 402 mg/L is that it raises the running cost of a tankless unit in Patterson rather than ruling one out. If you want tankless here, budget for descaling on a real schedule, ask the installer what interval the specific model’s manual requires at this hardness, and check what the warranty says about scale. Several manufacturers void on it.
What the number cannot do is measure your tap. It describes the City system over a sampling period that ran from 2022 to 2025. If the property is on a private well, none of it applies. If a decision turns on the exact figure, test the property. Our guide to hard water and water heaters covers the mechanism, and how often to flush a water heater covers what to actually do about it.