Curtain fullness
Fullness is the relationship between flat fabric and the finished coverage. It changes with the heading, the cloth, and the look you want.
Presets, not a single standard
A common published range is 1.5× through 3×. Some sheer treatments sit at the high end. Some tape systems publish other ratios, including 2.3× and 2.6×.
The suggested start on this site is 2×. You can type any ratio greater than zero. The calculator does not assign one ratio to every named heading.
Equal pairs
For a pair, the flat width is divided by two before counting fabric widths. Each side is rounded up, then the counts are added.
That prevents a result such as three widths for two matching panels. A single panel can still use an odd count, because there is only one panel.