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).
In your perspective does my baby bearded dragon look healthy or unhealthy? Does she look bloated? she is active and eats very well also she is in her basking spot all day