Maybe start at 39.5 or 40 degrees C, by 3:15 you may be close to 36 or 35.5 but your average temperature will have been good.
Having the exact temp all the way through is tough but I have had good results using this method. The biggest trick is being able to repeat whatever you decide on reliably.
I have also used the one sheet in the small tank in water bath method and it works but the temperature change is still pronounced.
One reason I don't sweat the temp change so much is that I don't have a calibrated thermometer.
Without a calibrated thermometer (which are normally quite expensive) you may be off 2-5 degrees any way, what really matters is good repeatable results.