Installing a gaugemaster station stop SS-2

[JohnCunningham]

Hi fellow modellers. I have never installed a SS-2 before but ready to give it a go. Question is, although the instructions are clear and understandable they don't tell you an approxiamate distance where to fit the reed switch on the track
Can anyone give me a pointer how far back, in distance , to fit the Reed switch so I can maximise the slowing down of the train and not make it too sudden a stop ? Thanks in advance