Is it a routed slot the same than creepage concept and how to calculate slot width?
Let red be the slot width and purple be the slot length, then


have the width be determined by the required clearance corresponding to the applied high voltage.
have the length and shape of the slot be determined by the required creepage corresponding to the applied high voltage.    


Choosing the width equal to the required clearance distance assumes the high voltages are on both edges of the slot. In your case, there is first a creepage towards the edge of the slot, the there is the clearance across the slot, then another creepage towards the other trace, making it actually extra safe/over-dimensioned.  

The blue line determines the real creepage.
I think most modern PCB tools can calculate track length for you, so you can draw a line or a track like i drew the blue line and add up their lengths.
Draw several blue lines to find out which is shortest and check whether the shortest blue line still satisfies the required creepage distance.