The key is to adjust your plan with new task additions and deletions whenever necessary. I do so about every week or two. For small task changes, make minor plan adjustments, but for major task additions or deletions, redo the plan. Because every project is unique, however, the only general guideline is to regularly check whether the plan represents the way you are actually working. If it does not, either change the plan or, if the plan is still the right way to do the work, follow the plan.