11.1.89.3   %MINUS

Decrement tag by specified amount, percentage , tag value or script variable.

# subtracts 1 from the value of tag named counter.

<SETVAL>counter=%MINUS 1  

# decreases tag2  by 1 percent of output span.

<SETVAL>tag2=%MINUS 1%

#  decrements tag2  by  value of tag1

<SETVAL>tag2=%MINUS @tag1