With new box turtle hatchlings, to be raised their first year indoors, under a Zoo-Med Reptisun 10.0 UVB bulb.... what distance should be maintained between the turtles and bulb?

The bulb is effective to 20".... so is 10" too close? There will be nothing between the bulb and the turtles (other than a hide box)..... and my set-up permits the bulb to be set at any desirable height.

Thanks,

Jay