On the bus if left to my own devices I will back calculate 1nm for ever 300' from the glide path intercept and 1nm per 10 knots speed reduction to S speed. So for a 1500' agl glide path intercept and initial speed of 250 kias I will use managed speed and begin speed reduction at about twelve miles.
If it's not a 3degree glide path I take that into account as well as any large variations or shifts in wind and ground speed.