It doesn't have to be directly in the middle but I would put it a little bit further in then the edge. It won't be less effective heating. The mat will restick and if your worried you can use foil tape to tape the mat to the bottom of tank. You might have to set the thermostat higher than 92 bc of the loss of heat through the glass, just make sure it is no higher than 94 inside the tank under the substrate on top of glass and between 88-90 will be ok for top of substrate.

Good call on getting a digital, just make sure you look it up or see if the package tells you how much it drops before it heats again. IMO I wouldn't want anything the drops more then 2 degrees but that's just me.

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