I don't have any information about this specifically but I'd try to imitate the UVI in their native habitat. Since they are woodland/meadow critters from Southern Europe (based on a quick search about them) I would find the UVI for their native areas and shoot for that. I would guess somewhere in the 1-5 UVI range basking area but I would provide a nice gradient with some total shade so they can pick what they want (similar to what you'd do with a dragon).