Hi,
I'm not sure I know the info needed to answer it yet as I'm not sure what you mean by 24" and 12" flexwatt.
It doesn't come in those widths as far as I know and if you mean different lengths of the same width heat tape then you broke the cardinal rule and used a different wattage for the different shelves.
But mainly I don't understand where the temps of 105 are when you say it is normally 90-95?
Also which tub she is in will affect it as the top shelves are usually a degree or two higher.
I think I'm understanding your probe placement to be on the 6th shelf from the floor? Is that right - because I don't understand the middle of the 4 shelves with the 24" if it isn't?
I'm just having trouble visualising the layout of the heat tape I'm afraid.
dr del