The debt ratio value of one or several issues on a code element, is obtained by this formula: 100 * estimated Debt of the issues / estimated effort to develop the code element.
The estimated Debt of the issues is the Debt value.
The estimated effort to develop the code element is obtained from this EstimatedNumberOfManDayToDevelop1000LogicalLinesOfCode value, and from the code element number of logical lines of code.
The value of this setting is just an estimation, in the short-term it is meaningless.
After a few man-years of development, this value is typically stable enough to rely on, for estimation purposes.
This estimation also needs to take account of the cost of writing unit-tests. The default value is 18 man-days which means an average of 55 new logical lines of code, 100% covered by unit-tests, written per day, per developer.

Lines of code as expressed by NbLinesOfCode is logical lines of code. This metric doesn't depend on formatting nor commenting and is inferred from # sequences points (gathered from PDB files).

Reference

DebtSettings Class
DebtSettings Members
ToDebtRatio Method