Here is a quote from John Palmers "How to Brew"
"A rule of thumb is one pound of liquid extract per gallon of water for a light bodied beer. One and a half pounds per gallon produces a richer, full bodied beer. A pound of LME typically yields a gravity of 1.034 - 38, as measured by a hydrometer, when dissolved in one gallon of water. DME yields about 1.040 - 43. These yield values are referred to as Points per Pound per Gallon. If someone tells you that a certain extract or malt's yield is 36 points, it means that when 1 pound is dissolved into 1 gallon of water, the gravity is 1.036. If that 1 pound is dissolved into 3 gallons, its gravity would be 36/3 = 12 or 1.012. The gravity is how the strength of a beer is described. Most commercial beers have an Original Gravity (OG) of 1.035 - 1.050."
So first off, you should make sure you are using the proper volume of water based on your ingredients. Secondly, its perfectly fine to add the water directly to the carboy as long as you keep everything sanitary. I would suggest boiling the water ahead of time and sealing it properly so that it doesn't become contaminated. Once you have poured your wort into the carboy it should be around 70F which the right temperature for a bacteria breeding ground.
