you want the thermostat probe outside of the tub, so that the snake doesn't accidently move it and then it will just leave the UTH running on full power (not good). your best bet is to get it in-between the tub and the UTH. there you can control the exact temp of the UTH. you'll just have to adjust the temp till you get the right temps on the acurite inside the tub.

hope that helps