Instead of using a lapping compound you could use adhesive backed sandpaper...I forget what grit I used, but guess it would be ~200 or 220 grit. Instead of trying to cut the sandpaper into a perfect circle for the lap tool, I just cut several small pieces and stuck them on there.
In addition to what OT mentioned about checking for square, I could visually tell mine was out of square by just setting it on a table..a small square proved what I was seeing.
I think I might have posted a couple of pictures of the job, inc. a pic. showing the high side before the job was finished.
