Those of us who have been adding DE to our sand filters recommend that you add a little at a time. Start with 1/4-1/3 cup, add it through the skimmer and wait 20-30 minutes. If your pressure doesn't increase by 1bl, repeat. Repeat the process until the gauge shows that 1 lb increase and that's the amount to use, however much it is. If the pressure goes up too much (like by 5lbs) you've added too much and the solution is to back-wash and start again.

How much will be right seems to depend on a lot of factors, including just plain randomness!!