Hey Sue,
Yes this can be a tricky one I know what you mean. Ideally you want to try to stay close as much as possible, if your opponent is driving or playing fast topspin shots the best thing you can do is try to block and stay close and look for an opportunity to counter spin or block your opponent out of position.
As you mentioned the only real time to drop back is if the ball pops up high and the opponent is going to smash or play a very aggressive shot, then you need more time to recover and get back into the point. So keeping the blocks low is important in being able to stay close and dropping back should be a last resort really.
A really good practice to do is getting an opponent or robot to topspin strong at you anywhere in the table and you have to practice blocking and staying close to the table. Some key things to remember are staying low with your body forward and keep the bat high when switching between backhand and forehand as well as keeping a soft hand when contacting the ball.
So really I would try to do the training so you get good at sgaying close and being more comfortable there and only dropping back as an emergency when the ball has gone very high.
I hope that makes sense. Cheers, Tom
