The 7 degrees is not exactly on, it is really 7.3. For that to be a good gauge you need to check the boiling temp of water with the same thermometer, or at least one that you have verified reads the same as your draw off thermometer. For testing the temp is not read until you have a full rolling boil, it is not as soon as you get a boil started.
I also used to draw a little heavy, but not a lot, then add sweet if I still had it, at the end of the season on final finishing I added permeate, before the RO I added distilled water, just a little at a time.
With my 2x6 I was always trying to do a constant draw, but it takes a rather fine stream and tiny adjustments to maintain the draw. I think the best I ever got without needing to close the valve was about 90 minutes, but 30-45 were far more common.
Now on an auto draw, I just verify the density about every half hour once I get it established for that day, it will and often does change thru the day, especially as the weather changes.