it's 40m to spawn Pokemon.
I think the tracker works that if a Pokemon is on your nearby list, it will display the closest Pokestop to it. So you can search in the vicinity of that Pokestop to find it. Theoretically anything on your nearby list would also display the closest pokestop.
The problem comes in when there are very little Pokestops near you so the nearest Pokestop won't narrow it down enough for you.
For me, however, I trip over a hundred Pokestops every day on the way to work, so it will work great.
The best part of trackers isn't included though - Trackers show you all pokemon in the area, not just nearby. The nearby is only pokemon within 100m or so. If a pokemon lasts 15 minutes, I would want to be able to see all pokemon within at least a KM or so. So the new tracking system is better but still meh.