If you call that result in your query "Unrounded", use this:
Rounded: 0.5*INT(2*[Unrounded])+IIF((2*[Unrounded])-INT(2*[Unrounded])>0.5,0.5,0)
edit: Oh, you got is sorted. Your way is better I guess. I don't really know many functions, I just worked out how you'd do it with what I know.