rounding up space

My body text is set with a 13pt leading but I also have other blocks set 
with a different leading. I want to add space before and after those 
other blocks such that their total size is a multiple of 13pt. Is it 
possible to do that? Of course, I don't know the minimum size of those 
other blocks in advance, since it depends on how many lines are generated.

Thanks,
Eric.