Launch heights vary with wind speed and direction.
With a 20kt wind straight down the runway, most winching sites can achieve something very close to the maximum height marked on the chart.
The other thing to note is the speed of transition from ground level to climb. It really is a matter of a few seconds from the launching glider being nowhere near you to it being at circuit height.
The most obvious thing a transiting pilot can do to decrease the risk is to pass the gliding field downwind, as winch launches will be fired off in the other direction.