As long as the room stays over 75 you should be fine with just the UTH. Closer top 80 is ideal, but if it drops a few degrees at night it won't be a big deal s long as the hot spot stays at 90.
So the zoomed uth can be adjusted to 90 degrees? And I also read online that UTH's can malfunction and get up to like 120, have you ever had anything that drastic happen? Also what substrate do you use?