For a more in-depth primer on radar horizon calculations in a variety of units, see
Computing Radar Horizon
The aircraft wouldn't have to lose very much height at that distance before it dropped below the radar horizon for a land based radar site at 200Nm out. Nevertheless, I'm confused - why can they not say 'it was travelling at [x] knots on vector [y] at rate of descent [z] before it vanished from our scopes'? That's what military radar installations are there for, surely?