I'd say realistic for a person of average height and weight 100% is the setting. It depends what you prefer. If you sit in there loosely or firmly in your seat with body tension, it'll appear different just as in real life. Nigel Mansell's head would sway a lot less than the average guy's due to his strong neck musculature. The motion sway does not simulate the cab sway which depends on the bogie suspension, but how your head would move, based on inertia and forces acting upon you. It's subjective and depending on the route. You'll find excessive sway on Main-Spessart Bahn which needs to be tuned down for example. On modern tracks driving a loco is usually a rather smooth experience. Or, technically speaking, a setting of 100% uses the default KSitting / KStanding / LSitting / LStanding XYZ spring values which are what the devs deemed realistic. Adjusting the slider multiplies these values. For me, more than 100% is too much.
200 is way too high for me. Driving a high speed train and going round curves induces motion sickness. I have it set to around 100 - 125 which works well enough.
I keep mince on 80% the forward and back motion on 100 feels too extreme for me. 80 is just about right.
