the trolling speed depends if you are using a "dipsy diver" (Luhr Jensen) or the Davis Instruments "fish seeker" (what we use) or the various other veins out there to get you to your desired depth.
the packaging (Dipsy) recommends "based on a troll speed of 2.5 to 3 MPH using 20lb Mono, the trolling depth will inc or dec by 10% for each one MPH faster or Slower.
the "fish seeker" packaging recommends "troll at 2 MPH use with light tackle between 6 - 20lb test"
unsure of trolling without these diver's but check the rapala website
http://www.rapalaworld.com/as rule of thumb we always try and measure against the shoreline and try to get our troll about walking pace, thats without the diver's on of course.
but once we discovered the diver's it sort of put us cranking the outboard up a notch
Trial and error i guess, some lures twist if going too fast, some don't swim correctly if going to slow.
what i normally do is hang the lure over the side and guage it from there if we are going to fast or too slow.
confused???