I'll have to guess couple of things but tell me if I'm wrong.
You are trying with last partition ? In that case merge it with middle one and than shrink that combined partition leaving unpartitioned space BEFORE it. That should allow you to merge those two partitions.
You can never get a round number because of the way it's calculated with bits, bytes, blocks and sectors and general architecture used in different OSs. Windows counts 1KB as 1024 bytes. Other OS may use 1KB as 1000 bytes, most Linux do it like that. Same as disk manufactures do so if for instance disk manufacture marks it 120GB disk, windows sees it as 111 GB or 114468 MB usable space.
What are you using for partition work ? http://www.partition-tool.com/ try this.