Question regarding negative number using a datasheet equation.
Slope compensation is required only when the duty cycle is greater than 50% to avoid sub-harmonic oscillations.
In the datasheet equations, a negative result for slope compensation results when VOUT less than 2X of VINMIN. The duty cycle is lower than 50%. So no slope compensation is really necessary.
For duty cycle lower than 50%, we generally can add a little slope compensation to make the current sense signal slope higher to reduce jitter in the switching.