Checking True

When I bought the CNC mill, I had no obvious way to check how accurate it was.  I had dial indicators, but no obvious way to use them.  That is why I was asking about dial test indicator holders recently.  But after a bit of thought, I came up with a solution.

That is a block of acetal with a 1/4" hole across, through which a 1/4"-20 screw holds the dial test indicator, and a 1/4"-20 threaded hole through which a long screw goes.  The screw end goes into a drill chuck.  There is less than .0005" movement per inch of X motion; perhaps .0006-.0007" per inch of Y.  Good enough.