Target a specific point or target on a large surface?

[[Mi...]]

I have a ⌀400 smooth machined cylinder in which the customer has placed a small 5.0x5.0mm square decal for an upcoming test. 

The decal is not scanning well enough to create any accurate measurements from.

Is there are way to mark or use some sort of pointing element to show the location of said decal?

 

 

 

[[Mi...]]

I ended up taping off 1/4 of each decal with tape, then sprayed the part. Took tape off and was left with a nice fairly sharp corner to be part of the scan.