Scale of the sonar screen

[MarkD]

I have Garmin Echomap SV sonar (which is, by the way, total crap in my opinion). The manual says I can adjust scroll speed or leave in auto which is matched to the boat speed. But this matching doesn't tell me anything because I don't have scale on the screen.
After a sonar detects fish, then this arc moves to the left of the screen. Let say I troll my lure 100' behind the boat. I want to know where my lure is according to the fish, if I passed it already and I should come back or there is still distance to go.
Could anyone tell me if it's possible to evaluate or not?
Thank you!

[Woodsman]

All scroll speed does is effect how fast the image moves to the left of the screen.
Slower scroll speed means the image moves slower to the left than with a faster speed.
The trick is to determine how far your boat takes to travel the distance that your lure is behind the boat.
Basically at 1 mph your boat will travel 1.5 feet per second.
Using your 100' back trolling at 3 mph it will take aprox. 22 seconds from when you first spot the mark for your lure to arrive at the place the mark was at the time you marked it.
When trolling you can not target individual fish but you can identify the general depth where they are hanging out.
Put your lure at the depth they are marking at and it's a good chance you will put your lure in front of some fish.

[MarkD]

Woodsman, thank you for your great explanation! Shame that I couldn't realize myself that I only need to count time regardless of the location of the arc on the screen.
I know it is very naive to target a single fish you marked on fishfinder while trolling but at Georgian bay you (or at least I) can troll for hours without marking any fish so when I get a reliable mark I want to be at that spot. Also I think that unlike salmons, lake trout is tend to be more solitary fish rather than school fish, but I might be wrong...

[Woodsman]

But remember that single fish you marked may have others 30 feet to the side.
Fish tend to group together by depth which effects temperatures, oxygen levels, light penetration and baitfish locations.
It's the depth you should target not an individual fish.
And during the time between when you marked the fish and your lure got to the area it was at, the fish could be a hundred feet away.

[MarkD]

Well, I generally agree, but I still think that it's better to return to the spot where you marked the fish rather than randomly go somewhere. Especially as you are saying there might be other fish nearby.
But I actually found right now that there is an option on the unit to mark a waypoint right on the sonar view. So my initial question looks even more meaningless.