The speed is an issue when you are using a flasher/hootchie or flasher/bucktail combo, and even then its not exact. For these setups the most common leader length used is 30 to 34 inches, but everyone has there preference. If you are in an area with alot of current, you have to adjust your speed/leader length accordingly. 2.5 to 3 knots just gives you an idea on about the speed you should go.

A knot and MPH are close enough that at slow speeds there is no difference you would need to wory about.

If you are using herring, troll fast that 2-3 mph when your looking for bait, but once you find it, troll around it as slow as possible as long as you get that good spin on your herring. Present the salmon with the easiest meal they can find.

Jig where the bait is, depth does not matter.