Drinking lots of water days before the test will not help. Drinking a lot of water on the day of the test is another story. Aerobic exercise can help but stop 4-5 days before the test.
I think your chances are average to below average as 230 pounds is a little on the heavy side for 6 feet and heavier people take longer to clear. You are doing the right thing by getting a home test.

Soluble fiber like that found in Metamucil can help a little. Take about 12-15 grams a day.