I just developed my own excel spreadsheet with the help of a few good brewing reference books containing the formulas I needed to calculate OG, IBU's, FG and ABV based on attenuation rates, amount of water needed, strike temps, etc.
I basically know what style of beer I want to make, and research/look up the typical OG/FG range, types and amounts of ingredients for a given style, etc. and then develop and fine tune my own recipe. It works as well as it needs to.