Discrepancy between probe temp and thermometer temp...

So, when you guys use charts like this:

http://www.kegerators.com/carbonation-table.php

To establish proper temperatures and psi settings, do you rely on the temperature from whatever digital or analog probe you're using on the thermostat?

I've had instances where there's been a 4-5 degree difference between what my probe is reading and what the small little analog thermometer that I placed in the freezer is reading. Am I getting inaccurate thermometer readings from the small analog one because it's sitting right on the cold floor of the freezer (or sometimes the hump)? Normally if the probe reads 45-47, it will read 42.

In a situation like that, I'm having trouble deciding what to put the PSI at because I don't know if the beer in the keg is closer to 42 or 45. Currently it's at 13 psi, and I don't have foaming issues or anything like that, but I want to make sure I'm shooting for the middle for ale styles of beer and not undercarbed.

Trust neither, pour two glasses of beer and check the second one with a trusted digital thermometer, I use my mash thermometer. Then adjust your controller accordingly. Although a 3f to 4f variance isn't really much to worry about, it will hardly change your final carbonation.

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I gotcha, I'll try that tonight - thanks