Radius gauge apparatus for vernier caliper

Simple apparatus to help measure radius on 90Â° 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.
Rest the object on the right angle edges.
Push the depth measurement pin till it reaches your radius.
divide the measurement to 0.414mm then add 4mm to get the radius. (remember zero measurement means 4mm radius)

alternative calculation:

zero the caliper at 8.24 (outer diagonal), then take the measurement.
multiply by 2.414