Let's suppose if your bike's consumption is 50Km/lit. (or you may call it X or whatever)
Case 1: So your bike hits the reserve at 2000Km (Take reading). You immediately put 5 litres in it. And it will hit the reserve at 2250Km (Take reading again). Right?
So you covered 250Km in 5 litres. 250/5=50Km/lit
Case 2: Your bike hits the reserve at 2000Km (Take reading). No pump nearby, so you just drive it on reserve for 50Km before you refill. What happened here... you consumed 1 litre from reserve also. Then you go and put 5 litres in it and turn the knob to "On" positon. 1 litre gone to reserve capacity automatically, leaving behind 4 litre for use which will give you 200Km more and your bike will still hit the reserve at same 2250Km.
So again, reserve to reserve 250Km in 5 litres.
(So yeah, your next reserve will the 2 or 3Km earlier as you said if you take the reading after putting 5 litres. You are supposed to take the reading when you hit the reserve)