How to determine low and high threshold levels of a Schmitt Trigger for a known input?
It's a judgement call.  The first thing you want to do is look at typical input waveforms.  The hysteresis offset needs to be at least the size of any temporary inversions in the signal.  For example, if the main signal was a 1 kHz &plusmn;8 V sine with 20 kHz 1 Vpp sine on top of it, then you would need at least 1 V hysteresis offset to not trigger on the 20 kHz component.

The main tradeoff is that wider hysteresis offset gives you more noise immunity, but also increases the chances you wont see a real signal that is a little out of spec.  Given no other information, start with the hysteresis levels at 1/3 and 2/3 of the input signal range.

You say your input is a "pulse train".  If the input is really pulses, then you may not need hysteresis at all.  Perhaps all you need is good gain about some convenient center point.  Hysteresis is good for two things: Immunity to some noise added to a larger binary signal, and dealing with arbitrarily slow slew rates.  "Pulse train" suggests you have neither problem.