Just a couple more thoughts, Ben. As you mentioned yourself, the lake levels info is readily available in many forms, including websites and phone apps. For now, I would certainly concentrate my efforts on your original idea ... water temp, which is NOT readily available. Don't dillute the formula, in other words.
About location of the sensors ... If there was any way for them to be in the shade 24/7 (inside a boat slip, maybe?), that would eliminate the potential of "false" readings caused by sunlight. I would also suggest that the probes be placed over water as deep as possible, so as not to be affected by the tendency for shallow water temps to fluctuate greatly.