Heat probe placement!

I have a temporary setup with a zilla heat mat connected to a Vivarium ve-200. I'm not sure where to place the probe exactly.
Any answers will help I'm getting 98 degrees inside but 87 outside on mat.
Temp set to 89

Thank You

Place the probe outside the enclosure between the UTH and the bottom glass. Check out "jfreels glass tank setup" sticky thread on this page. Shows you exactly what to do