If it's shining through a screen then 10'' is definitely appropriate. The fixture can face either way, that won't make a difference for the reflector.
I just took a measurement from my Arcadia T5 to give you an idea of the output at distances. The output is pretty similar to the ReptiSun 10. At 10'' I read 4.6 UVI. Generally 3-6 UVI is a good range for the basking site. I get a UVI of 6 at 8'' below the bulb. This is directly below the bulb, the output drops as you move to the sides so that provides a nice gradient as well. If I put a fine mesh screen in front of the light, at 9'' I read UVI of 3 and at 4'' I read UVI of 6. My point is just that you are not causing any issues with having the bulb at that distance.