I will not discuss tactics, sensor can be used in many ways, it's up to you to decide what's work for you.
When you turn radar on, aircraft in radar cone detect you emitting. When you radar lock a target, he has lock indication. When you fire R-27R/ER at someone, he has launch indication. If you lose radar lock, IRST will try to relock automatically.
When you search via IRST, there's no indication target is visible/locked.
There is pretty neat integration of sensors at Su-27, I suggest you to play around and test how it works.
"Expected target range" will help the radar system to calculate altitude range for given range. Nothing more, nothing less.