Determining the percentage of emails landing in Gmail's promo tab involves using inbox placement monitoring services, seed lists, and testing tools. While direct measurement is difficult due to privacy restrictions, these tools offer estimations. Many experts suggest that focusing on sending valuable, engaging content to a clean, segmented list, and maintaining a good sender reputation is more important than strictly avoiding the promo tab. Some argue the promo tab isn't detrimental, as users often visit it specifically seeking deals. Key strategies include improving email content, ensuring proper authentication (SPF, DKIM, DMARC), avoiding spam triggers, A/B testing subject lines, and regularly cleaning email lists.