Drifting-Uphill,downhill & level

Can anyone tell me how an engineer determines when to notch down the johnson bar and how much monentium his train will have so as to drift into a station or siding. I am thinking more of small motive power such as ten wheelers, consolidations and small mikes. Engineers faced differant grades such as uphill situations similar to Chama, Lizard Head and Larch. Downhill locations such as Ophir or Webster. Level situations while common have certain locations (ie Sapinero) where long trains were common and I wonder how an engineer of say a small consolidation determined when he could cutoff power and roll in. Thanks.