I also use ibrewmaster. I brewed a recipe that said the OG should be 1.066 at 70% efficiency. I put it in ibrewmaster and it came out 1.058 at 75% efficiency. I was concerned but brewed as planned. After wort boil and cool down was dead on at 1.058. IBUs were off a little from the recipe but it tasted great. Ibrewmaster has been pretty accurate for me.