Sorry I haven't read through the whole thread just this last post BUT those stick on thermometer are basically useless for this purpose . To get accurate readings of the warm section and cooler side you need those digital thermometers with a wired probe and/or a digital temp gun . The probed ones can be bought off eBay for as little as 3 dollars and a digital temp gun can be bought off eBay / Amazon for about 13 dollars ..
You may already have some of them as you mentioned the temp of the substrate but if the stick on ones are all you are using then the true temp of the substrate could easily be over 100F