I calculate conversion rate based on the number of people that hit the course page. That's industry standard. My goal was to test how many people that read about my course signed up. The blog post drives traffic to the home page, but I don't count blog readers if they don't hit the home page because that wouldn't be testing the right metric.
Conversion rates vary by business. A media company tracks the number of readers that click an ad. A mobile app tracks the number of visitors to the home page and download the app. This was the metric I picked to see if people were interested.