The feedback resistor should be a bit higher-value than the input resistor. The closer they are, the more hysteresis there is.
The resistor from base to ground centers the hysteresis. To center on half the supply voltage, it should be half the value of the feedback resistor.
2017-10-11: Added some optional output loads because they have an effect on the hysteresis. Being in parallel with the resistor from base to ground, they change the center of the effect. I guess a practical design would use another transistor as an output buffer unless its output load is known. Then again, the effect is small unless the load is large. 50k hardly makes a difference.
