The process of model building followed by refinement can seem to repeat endlessly. Somewhere it must end. A good time to end would be when the new model, ready for model building, is so similar to the one from the last round that the new map will show nothing new. The Fobs are always the same -- the new map will be distinguished by the difference in Fcalc and calc (the calculated phase). In my experience, I have never found a difference in a model when the R value between the two sets of Fcalc is less than 3% and the r.m.s. phase difference is less than 10 degrees. These parameters should be monitored each round of model building and can be used to judge when refinement should stop.