You are over simplifying the process. In reality you need to measure and optimize each of these 4 elements:
1. Quality of your subscribers.
- Determined by the quality of your traffic
2. Then you have to measure the email open rate.
- This will determine if you are hitting the inbox or spam and the quality of your subject line.
3. Then the email click through rate.
- This is determined by the quality of the body of your email
4. Then the conversions.
- This is determined by the quality of your landing page and offer.
I explain what I do in more detail here:
https://www.blackhatworld.com/seo/why-you-are-failing-with-facebook-ads-and-dropshipping.1291302/ this process applies to most b2c niches I work in.
Usually you are in the thousands of subscribers not hundreds before you can measure the lifetime value of a subscriber. Although it differs by niche, on average I found it takes about 100,000 subscribers for you to optimize your funnel.
A one off conversion rate is simply a reflection of the above and that specific offer. Not necessarily the list quality.