I have a floating Thermometer, I leave it in the plastic tube, just float it in there with sanitizer and then strap it to the Primary with the brew belt.
That way I know how much heat the brew belt is providing aprox what the wort temp is in the fermenter. It's still a few deg low most likely, but as I don't let it get over 70. I'm in the ball park.
I record the temps twice a day (morning and night) to watch the fermentation accelerate then drop off. I then know when to plug in the brew belt to keep temps up to continue fermentation and avoid the stalling of the yeasties.
Then when it falls to 68 deg F with the brew belt on, I turn it off and record the drop to room temps (total time 8-10 days) Then I'm ready to rack to 2ndary and record the SG.
This way I'm not opening the Primary to get samples for SG testing. The yeast knows when it's done. The SG is just to get the result and figure ABV.
Works for me, but there are always other ways of doing things. Beer brewing is an individual process thing, and we all have our own quirks and it still makes darn good beer!
Cheers