What temperature is it safe to take a bearded dragon outside?

I don't want her to be too cold. I've taken her outside a few times but only when it's been really really hot. I'd like to get her more sunglight summer. Also, the woman at the pet store sold me one of those iguana leashes. She says her neighbor uses it for her beardie. I have it on Oly, It does get small enough. I'm just curious if anyone has had any problems using the iguana leash on their beardie. Whenever I take her out she sits on my lap without budging. But I want to make sure she's safe in case she decides to make a dash.