I do extract brewing and make 5 gallon batches on average. I have a 10 gallon brew bucket with a thermometer built in, but it only reads if i can fill the bucket with around 4 gallons of water. In all my kits, they say to add 2.5 gallons of water and then add your grains, etc. Why only 2.5 gallons? Can I not just go ahead and add 4 or 5 to start with and steep my grains in it?