Where to measure basking temp?

I currently have two baby red eared sliders and recently switched to a mercury vapor bulb for basking versus an incandescent and a UVB. My issue is that with this bulb I am having a little difficulty measuring the basking area temperature...

With my previous set up probe placement didnt make a difference much, but now there is a 4 to 5 degree variance between if I have the probe touching the basking rock versus hovering over it in the air. Obviously the new bulb is just heating up the actual rock more than the previous one... So my question is which one do I measure: the surface temperature of the basking platform or the air temperature of the area?

At the moment I am adjusting temp based on the air temperature a half an inch above the basking platform. My logic is that the turtles move into the water if they get too warm due to the light on their backs, or even sit on their log that isn't under the light; I'm not worried about them overheating, ut I am worried about the air in the tank being too cold and causing respiratory issues. What do you guys think?