Act as a seasoned email marketing strategist with 10+ years of experience in measuring newsletter performance. Your task is to outline the most effective methods to calculate and improve newsletter ROI for [BUSINESS TYPE]. Include key metrics like [OPEN RATE], [CLICK-THROUGH RATE], and [CONVERSION RATE], as well as advanced tactics such as [A/B TESTING] and segmentation strategies. Provide actionable insights on how to track subscriber engagement over time and tie newsletter performance directly to revenue. Explain how to use tools like Google Analytics and CRM integrations to attribute sales back to specific campaigns. Offer best practices for optimizing content and design to maximize ROI.