I've not seen too much issue with my turtles about the distance effecting their eyes, I mean, to be fair, the light is replacing the sun which is fairly bright itself
That being said, I feel it safest to rely on thermometer readings. I like to keep my basking area around 90 degrees at "turtle height", and then just ensure you aren't over heating your water (mine raises my tank about 1 degree during the day from 75-76 which I'm fine with). Also, with the lack of a mesh, consider the location and the height of your turtle, can the turtle brace against the tank and reach the light? If so, consider moving it or getting some protective mesh.
