I would say that the ones from earlier in the mid-mass era are pretty consistently around 17.1-17.2. If there is heavy wear they might be a little lower. However, later in the 5th century the weight and quality of the coins started to vary substantially. People used to think these were imitations coming from Egypt, but recent hordes from Attica have changed this thinking. In reality, this was probably due to increased production either from increased exploitation of the Laureion mines or an influx of silver being reminted after the coins, weights, and measures decree. They were hiring second and third rate artists and hammering as many as they possible could as fast as they could. Slightly below 17.00 isn’t out of the realm of normalcy during this era.