I'm talking about the one you enable via Poser Main Menu > Display > Guides > Focus Distance Guide, the one in the screenshot below.

The problem is that most of the time I want to check the distance to an object which is neither at the viewport centre nor on the diagonals, so I need a guide that includes more circles and/or radial lines.

I assume that the Focus Distance Guide is either a geometry mesh or a texture image on a flat plane, but can't find either.

Any hints ?

BagginsBill has an alternative to the default camtarget.obz. You can find his here.

Navigate to your Poser 11 Content folder and in Runtime/Geometries/Camera you will find the original which you should back up and then replace it with BB's. It is not exactly what you've asked for but is much better than the default one. I've replaced it in all versions of Poser that I use. It looks like this:

Excellent !

Thank you Kalypso, and of course Bagginsbill.

(I should have guessed that camtarget.obz was the Focus Distance Guide - if I'd extracted it and looked at it in an editor it would even have told me so)