Ah, water, elixir of life and bane of many a coldworker. Water can make or break how well your diamond disks work, and knowing how much water to use for particular steps can be the difference between joy and heartache.
Rougher diamond disks like the 45, 60 and even 80 grit will initially need quite a lot of water to keep the ground glass off the disk as you grind. Finer grits require less water to work. As diamond disks wear down, they will all require less and less water to operate efficiently.
Water usage is a constantly changing medium on a diamond disk. It's a bad idea to find one water setting for a disk and leave it there. As the disk wears down, you will begin to find that you'll end up hydroplaning on the disk (riding along on a thin film of water) instead of grinding on the diamonds.
A good rule of thumb for water usage is to lower the amount of water on the disk until you see a slight buildup of white ground glass developing on the edge of the disk. Then bump up your water just enough to make that buildup go away.
This is something that you'll need to do with each grit disk and even with each usage. After a while, you will get a good "feel" for the amount of water a disk needs to efficiently grind.
Be sure to wash off your disk thoroughly after each use to prevent any ground glass from drying on the disk. If left unchecked, the ground glass can eventually cover the diamond and make the disk useless in that area.