What kind/brand of UTH do you have. I have a ZooMed pad for 10-20 gallon tanks and it has gotten my substate up to 98 along with a 40 watt black night bulb. Needless to say after a few weeks I've realized the importance of a Thermostat to regulate the power output of my UTH, hence regulating the exact temps in my tank. I don't believe there are any UTHs with build in thermostats to regulate the exact temps (although that would be a great idea for any of you engineers out there).

Your probably right about the inacuracy of your current thermometer if you got a cool side reading of 100 when there's no heat source over. Personally, I would leave the 60 watt on if it brings your temps up to 88-89 which is a much healthier range for than 72-74.

Don't worry about a pic of your tank(unless you want to post it, I did when I was first trying to set up and it helped me). I think we all want to see the little baby though