In my current build of George's four cylinder, I needed to mill a tangent to two different lenth radii in an internal pocket (Red line in first picture). Initially, I was going to sweep them with an indicator; but quickly realized that, one it would be a pain in the bottom; and two, it most likely would be prone to error. What I came up with was the following:
1) Place two ball bearings against the radii.
2) Place a tool bit against the bearings.
3) Set an indicator against the tool bit.
4) Squeeze the tool bit and move a bearing back and forth watching the indicator.
5) The bearing is set when the indicator indicates it's lowest point of travel.
6) Repeat 4 & 5 for the second bearing.
7) Indicate the tool bit as if indicating a vise jaw and adjust the part until the indicator reads zero.
Once the tool bit indicated zero, I proofed the method by removing the bearings and starting over. The tool bit indicated zero without any further adjusment of the part.
-Bob