Ok, I've been chipping away at this.

The Employ speed % started to drop as I approached 30 Far Afield

F/A 24 96%, F/A 27 93%.

Employ Stealth remained 100% @ 190 Shadowy. I have several equipment options to go higher so I can get some extra info.

At 30 F/A The only option becomes "Sight one of the castles on the roof" which has no check and gives you1 x Sighting of a Landmark on the Roof (which you can cash in back in London)

I'll keep going to 55 Far Afield and keep track of the checks and post here if that helps