What is the correct way to write a ratio as a calculated field?

Aggregate both sides before dividing: SUM(Revenue) / SUM(Sessions).

Writing Revenue / Sessions computes the ratio per row and then aggregates those ratios, which weights every row equally regardless of size and produces a subtly wrong number.

It is the same trap as AVG(a/b) versus SUM(a)/SUM(b) in any other tool, and it is the most common cause of quietly incorrect metrics in Looker Studio reports.