Speed depends on the specie...and what they want. Sometimes slow is good. Sometimes, it's more speed they want. Personally, I always vary my speed--until I land on the "speed of the day".
But typically, the trolling range that you listed is indeed the "sweet spot". I recently bought one of these (see pic), to try with my 9.9 motor. Won't be able to try the "Happy Troller" plate, till next Spring. But, sure am looking forward to it.
Lots of peeps, though, use milk buckets and other items with pretty good success. Hope it works well for you. Let us know, how you do.
P.S. If you're not already doing so; remember to troll in an "S-Curve" pattern across the water. Far too many peeps, troll in a straight. An S pattern, drops the gear deeper on the turning side--and raises gear on the other. Sometimes, it's such a change that gets bites!