Can I make a new column based on a ratio of two metrics?

No, but you can request a custom metric through the Investors’ Library. See this help documentation for instructions on requesting a metric. Custom metrics are only available to Premium members. However, if they are popular enough, certain custom metrics will be promoted to the main product.

For screening purposes, Premium members can construct new metrics out of existing metrics using freeform equations. Instructions for creating freeform equation screeners are here.