Subtract a dgree for rectal? I thought rectal wasa the most accurate. I have never heard of subtracting a degree for rectal.
Once I took DS's temp undersarm and then rectal and there was a 1.5 degree difference.
Overall though, it doesn't matter the exact temp if you get a roundabout temp. Look mostly at how babe is doing. Treat the baby, not the temp. I say this, but I am a bit (well, a lot) obsessed with the thermometer myself as I am always just really curious.