When I'm feeding balls for a warm-up, I'm hitting the same shot over and over. It doesn't matter where my opponent is standing. I'm calibrating my aim to a four foot box in front of the hash-mark on the baseline. I'm just using the speed of gravity for pace; not trying to over or under swing, little if any bicep or tricep involvement, as flat as possible.
It doesn't matter if my opponent is at the net, at the baseline, in the parking lot or at the bar. The only variation would be when my opponent at the net, points his finger up, meaning he wants overheads. And then of-course taking serves.
A tip to keep the balls from going into the net or short, is to aim for the center of the opponents back fence net-screen. Usually there's a horizontal line in the middle, where two screens seam together. If you aim for this area as a horizon, the ball will stay high enough to clear the net and gravity will keep it inside the baseline. This will also smooth out your swing, holding the finish until the ball bounces on the other side.
I've only lost one warm-up.