Is is possible to format the number that displays in Tag Panel? Currently there there is no pattern and the numbers don't match those listed in Length.
Have a look at the extended tag-view (ALT-t) of a file and best give us a screenshot.
The "length-field" has the paceholder %_length% and you cannot add this to the tag-panel, because the tag-panel is reserved for editable tags. %_length% shows the technical info of the file, not a tag-field.
If you have another field called %length% it is a user-defined field.
Not in the tag panel.
But if you select just a single file then you see the length in the status bar.
(If you selected several files, then you would not see any data in the field for length anyway but <keep>)