I would use a piece of unprinted decal paper and some liquid decal film, (such as Microscale's), coat paper with decal film, let dry, use a circle template or stencil to paint white circle, coat again and let dry. Apply decal and use decal setting solution to snug it down over any protrusions.

The Italians probably hand-painted the circles so if it's a little sloppy it should be ok I'd think? 

You could paint the turret white first and then fit the template over the details. Then paint with the top coat.  I did this on my SMS Konig battleship turrets and it worked just fine.