Based on your description, it seems that you're on the right track to find the number of Unique Visitors for the Last Month who have previously visited your site within 2 years.
To answer your questions:
Your approach is mostly correct. However, there is a minor issue with your segments. Segment A should be the visitors who visited your site in the last month, not the last 2 years. Segment B should be the visitors who visited your site within the last 2 years and excluding the last month. Segment C should be the result of including Segment A and excluding Segment B.
While your current approach is not wrong, there is a more straightforward way to achieve the same result using a single segment.
Here's an alternative approach:
Create a segment, let's call it "Returning Visitors Last Month," with the following conditions:
a. Date Range: Last Month
b. Then, within the segment builder, add a container with a "Visitor" scope.
c. Inside the "Visitor" container, add two conditions:
i. Condition 1: Select "Visit Number" and set the condition to "Greater than or equal to 2."
ii. Condition 2: Apply a date range for this condition: "Last 2 years" (excluding the last month).
Now, apply this segment to your table:
a. Column 1 - Unique Visitors (no filter)
b. Column 2 - Unique Visitors (filter - "Returning Visitors Last Month")
This approach directly provides the number of Unique Visitors for the Last Month who have previously visited your site within 2 years, without having to create multiple segments or subtracting values in the table.
I hope this helps! Let me know if you have any further questions or need additional clarification.