Unless I'm misunderstanding things:
=IF(MOD(x,FLOOR(x,1))>=0.51,FLOOR(x,1)+0.9,FLOOR(x,1)-0.1)
I'm no good at writing Excel macros, but you can put that directly in the cell you want to update and change x to the cell number, i.e. A1.
EDITED: 9 Dec 2009 21:51 by MATT