indent: add 'const', rename variables, reorder formula for tab width

Column counting starts at 1.  This 1 should rather be at the beginning
of the formula since it is thought of being added at the very beginning
of the line, not at the end.

When adding a tab, the newly added tab is added at the end of the
string, therefore that '+ 1' should be at the end of the formula as
well.

No functional change.

(rillig)