Are you using a controller to monitor temperature, and if so are you using a thermistor (probe that came with the controller). If so what I do is mount the thermistor to the outside of my glass fermenter using bungee cord and and put a blanket between the sensor and the bungee. This still will not give you the reading of the beer inside the fermenter. You will have to offset it a bit with trial and error depending on your set up.
Alternatively you can purchase a thermowell that protrudes into the fermenting beer, and place the thermister in the socket. I have no experience with this, but I would imagine they still would not give you absolute temperature, as there would be some temperature loss between the core of the beer and the sensor.
Not knowing your set up, or ambient conditions, and assuming you have your fermenter in a fridge I would guess that you would have to set the temp lower during high krausen and then increase as the krausen settles.
But honestly, I would relax and not worry. I would set the temp to 66 and forget about it, experiment with the beer and see what works for your set up. If it's overly fruity/estery lower the temp next time.
