Exactly what I do. The cap on the filler neck states that max capacity for the reservoir is 4 liters. A gallon is 3.78 L. If you look at the manual, it states the light comes on when that 4L is just about up. It would be nice if you could see exactly where yours was at just in case you wanted to top up, but unless you want to take your right fender off, that isn't happening. As with the rest of this car, it's usually best to listen and wait til it tells you something needs changing or is off. There's obviously some things it doesn't monitor, but for the things it does, it usually knows what's going on pretty well. It's interesting to read up on this car's computer systems.

Sorry, went off on a tangent there. Yeah, don't fill it up unless the light comes on. Disaster will ensue.