Hi Geoff,
If you want to keep within ISSF rulings the table can be no more than 1m high. Many tables in ranges are not that high and so people tend to use their pistol box or other such box to bring the height up a bit. So long as the combined height of the table and box does not exceed 1m that is fine.
I found this out when I did my first shoot competition at Bisley. I always rest my pistol on the target changer at our club and when I got to Bisley, no target changers - it really threw me for a while until I saw people using their pistol boxes for extra height.
Regards
Mark