Radius gauge apparatus for vernier caliper

Simple apparatus to help measure radius on 90° edges, with a vernier caliper. This size can measure between 4mm-20mm.
I needed to measure the radius on various phones and tables, and found out there was no simple solution, so i modeled this, and printed it on reprappro mendel tricolor.

updated instructions (thanks to krosstoph for pointing it out)

How to use (see pictures)
- insert the depth measurement end of the caliper in to the slot. Zero your caliper at 9.89mm for convenience.