That is how most do it. I vorlauf until the bed is set and the wort is relatively clear, then I slowly start running to the kettle. When the level hits the top of the grain bed I start the sparge water. I keep an inch or so on top of the bed until I have added my calculated sparge volume. Then just run it dry or until you reach your target kettle volume. However, If you decided to just heat an abundance of sparge water and sparge until you hit your target kettle volume, the only thing you lose is propane. It makes no difference if you leave water behind. The things that effects your sparge efficiency is things like avoiding channeling, getting a decent first wort gravity (19-21 Plato, mostly determined mostly by your liquor to grist ratio and how much foundation water you add the the LT), running off slow, and doing your best to not mix too much the strong runnings in the bed with the sparge water (meaning lowering the level until you reach the top of the bed before starting the sparge). You want the concentration gradient between the sugary liquid trapped in the grain and the sparge water to be at its greatest to pull those sugers into the runnings.