I am attempting my first higher gravity mash tomorrow, shooting for around 1.087-1.090 OG. I batch sparge and usually have overall efficiency of 75-80%. I calculate my grain bill using 75% and usually hit my target OG or overshoot by one or two points. These beers have been in the mid to low 1.050's.
Does anyone have a rule of thumb for lower efficiency at higher gravity? Should I try for 70% or even 65%? Any advice is appreciated.