I use a 1 minute prewet on the Jobo drum processor, but for tray processing I don't.
I measure to the square nanometer and to the 0.001 cc in volume. NAH, just dump and stir in some extra as you go along. After you have done it as long as I have, you develop a 'feel'. Its like swimming or riding a bike. Once you learn, you just don't forget!
The kit I use is the RA-RT developer replenisher. It is not the 1 gallon kit.