I use the Brew365 calculator and I have all my variables dialed in on my system. Take very careful and detailed notes on exactly how much water you use and have at each step of the process. Then, after a couple brews you will know what your variables are. For example, the default "Equipment Loss" variable of 1 gallon is way too high for my system. If I use that value, I'll end up having a huge sparge amount and not being able to take it all and then having a big loss in efficiency. Instead, I use 0.25 gallons. Also, my percent boil-off is always 13%, not 10%. So the first time you use this calculator try to estimate what each value should be (except for the relatively constant ones like wort shrinkage), take very careful notes, then adjust them accordingly the second and third time you brew. After using this calculator about 30 times, I always have the amounts I need. (Also, this is a side note, but another thing that can significantly affect your yield post-fermentation is whether you use hop bags or not. Definitely use hop bags, if you don't, the sediment can congeal with the trub and end up stealing a lot of beer away from your final yield.)