Subtracting one from the measure number

[Phil Holmes]

I've got a piece of music that has a dotted bar and a (hidden) time 
signature change at a place where the bar number is not incremented.  I know 
I could explicitly set the bar number to the new correct number (calculated 
number minus one) but is there a way of subtracting one from the existing 
bar number?

Thanks.

--
Phil Holmes