[How do I?] Radius gauge for 'large' values of radius

[THX-1138]

I don't have a micrometer that big, so I used a decades-old outside caliper to free-hand it as best I could. Keeping it centred in two axes was a chore. It appears to be constant-radius, so spherical at 4²³/32" Ø.

I really do need to get a larger capacity micrometer.

Thanks for the help!