you'll get a very unreliable measurement and correction at mash temps. I take a sample to the freezer. It only takes about 10 minutes or so to cool to 80+ Then I take the reading and dump back in the kettle. I usually have the sample in the kettle before the boil begins.
Really, though, I'm wondering why I even take that reading. I'm not adding DME no matter what, so I really get nothing from the reading other than confirmation conversion happened. I can taste the wort for that... I may just stop taking this pre-boil reading.
Doh! After I posted this, I just happened to think about this while sitting at a red light. Why not just cool the sample to the post boil/cooled temp. I'm writing down everything just to try and get a handle on the whole process. I sampled gravities from each running, just to see how much sugar I was getting out of the mash and subsequent sparges. At this stage, I'd rather have too much info than not enough, then sort it out later in front of the tube with a cold one.