Inherent hazard with Kings Cross is the pointless-ness of planning.
I've found that I usually knew a few days ahead when and what time I'll be at Kings Cross. And the WG's that show up in the search result on the "day of planning" are very different from those that are available on the "day of the deed".
Not sure why, but perhaps in the spirit of the transience of the station itself, the turnover of WG's in the area seems high.
Always find myself doing a last minute vivastreet search, VS planning to buy a on-the-run burrito dinner VS, catching the earlier train home! Then VivaStreet fear kicks in, leading to abandonment, unhealthy burrito, delayed train, and staring at giant train departure screen for the next 2 hours, all the while feeling hornier and hornier... Quality certified Standard operating procedure for KGX!