When cooking beef brisket in a slow cooker, the amount of water you need will depend on the size of your brisket, the size of your slow cooker, and whether you're adding other liquids or ingredients to the pot. Here's a general guideline to help you determine how much water to use:

1. **Minimal liquid**: Slow cookers are designed to retain moisture, so you typically don't need to add a lot of water when cooking beef brisket. Start with about 1/2 to 1 cup of water or beef broth in the bottom of the slow cooker. This will provide enough moisture to create steam and prevent the brisket from drying out during cooking.

2. **Size of brisket**: If you're cooking a larger brisket (e.g., 4-6 pounds), you may need to add a bit more water to ensure that there is enough liquid in the pot to cover the bottom and create steam. Aim for enough liquid to come about halfway up the sides of the brisket.

3. **Other liquids and ingredients**: You can enhance the flavor of the brisket by adding other liquids or ingredients to the slow cooker, such as barbecue sauce, Worcestershire sauce, onions, garlic, or herbs. If you're adding additional liquids, you may need to adjust the amount of water accordingly. Keep in mind that the brisket will release some of its own juices as it cooks, so you don't need to add too much extra liquid.

4. **Be mindful of overfilling**: Avoid overfilling the slow cooker with liquid, as this can cause it to overflow during cooking. Make sure there is enough space in the pot for the brisket to cook properly without spilling over the sides.

5. **Check the manufacturer's instructions**: Refer to the instruction manual for your specific slow cooker model for any recommendations or guidelines regarding the amount of liquid to use when cooking meats like beef brisket.

Overall, start with a conservative amount of water or beef broth in the bottom of the slow cooker, and adjust as needed based on the size of your brisket and any additional ingredients you're using. The goal is to provide enough moisture to keep the brisket tender and juicy without drowning it in liquid.