I generally think around 84-85 is what you want... But usually you want your basking light in general raised off the screen. So I would opt for the 75 watt and use blocks of wood or smaller cement laying bricks (usually I break them) with allow you to lift it just enough. If you lower the basking branch I wouldn't worry too much about uvb affected that quickly considering the most you would lower would be around an inch or so. UVB in my opinion, is not meant to be as strong as it is in a cage as it is out in the wild. The thing is the natural sun I think just has healthier levels of what they need from the uvb rays. Honestly the strongest basking rays during the day usually only lasts for about 3 hours. If you are using reptisun uvb I have noticed more sensitivity to these, even in the lower 5.0 range, than I have in the 6% or even the 12% arcadia bulbs. In my opinion this means there is something different and less good about the reptisun. But I actually haven't been able to put this theory to test.