I found that it was much easier to get consistently accurate readings by making a simple jig to attach the dial indicator to. The dial indicator is a popular one made by Mitutoyo, a series 2, model 2417. These can be bought used on eBay from time to time at a good price if you shop carefully. Mine cost me $20 delivered. I added a 1" extension rod to make the job easier.

In hindsight I would have made the piece of plywood in the back go the entire length of the top piece of plywood. This would have made it a wee bit easier to hold in place, IMO. I've got some nerve damage in my right (dominant) arm, so the jig was a big help to me.