There has been much research and discussion on the principles of standards-based grading, and there is a growing consensus of best practice. Even so, the actual process of implementing standards-based grading at a school or district level can be a significant challenge. There are very practical questions that remain unclear, such as how the grades should be calculated. Common methods include a simple average, averaging only the more recent scores, mathematical models of growth over time, and basic teacher judgment. It is difficult to choose a single method that is justifiable in all circumstances. This article proposes a new method, the history-adjusted true score, that can be applied in a wide variety of situations. A rigorous method of evaluating each grading method is also described.