A 10.0 is rated to output uvb 24" from the bulb, 5.0=12" Thats what the website says.
Ive used a 10.0 with no issues. Just make sure there is cover, in case its bothering the cham.
Ive used 5.0 with dense foliage through a screen, and it seems to work also.
Just judge the situation, and keep an eye on the cham.
A uv meter will take all the guess work out of it.