It depends on what you are trolling? (bunker spoon, umbrella, jigs, etc?) Like any trolling lures - Each lure requires different speed and some needs minor adjustments. Also, each boat/motor are different ?
Therefore, the best way to find the proper speed is by idling your motor/s low first and place your lure in the water (to observe the proper action) and increase the speed until desirable lure performance.
Once you are satisfied with the action ?that will be your "BEST" trolling speed!