I am making a 10gallon batch of a Belgian Wit with an OG 1.045. I have 1 tube of White Labs WLP400. I need to know how larger of a starter i need to make to cover 10 gallons.
Roughly speaking, about 2.5 liters. See Mr. Malty's yeast calculator. You'll need to know the production date of your yeast, and select how you make your starter (stir plate, simple starter, etc.).