Tools from UMM ™ 3D Center Locating Tool.
This precision tool allows you to locate the exact center of a disk or square.
Slide the piece into the 90 degree end and draw a line along the guide.
Rotate the piece and repeat the marking. Several marks will intersect at the center of the piece.
This has applications in all types of modeling.
Highly Recommended!

Other versions available:
MN074 UMM™ 3D PECO Center Locating Tool Jig (XL)
MN073 UMM™ 3D PECO Center Locating Tool Jig (S)