Cool side Ambiant needs to be 78-83...
Hot Side Basking area 88-91...
as far as the thermometer on cool side have it hanging down the back corner 1/2 way between enclosure floor & lid..
As far as a dimmer, thats what some Lights have in houses.. It dims the light (cutting some electric current) well your going to dim your heat bulb/CHE or whatever until you have the temp in the enclosure you want, this controls it from running Wide Open.
(what you mean about your "Cool side hot temp & Ambiant air I dont understand)
Your coolest temp should be 78... If your temp is fluctuating down to 73 that is not good. Don't forget thermometers are off too so that could accually be 68/69 degrees....however on the bright side thats why i like Belly Heat 24/7.. They are smart enough to stay on the heat mat when needed..
To solve the fluctuation in temps (cant remember what enclosure you have) block off any screens partially with Tin Foil. Ive also used a towel but not near a Heat Bulb ect....
in the future invest ($195) for a "herpstat 2" possibly. If your staying in the hobbie that controls 2 heat sources. It also has a alarm incase a heat source stoped working. Here in NY the night temps can go down to -15 outside... If my UTH or RHP burned out the alarm would go off when it shut off or went under 85 degrees... Herpstats are expensive, but they are the best out there i think...
Also FYI , APCAGES has a sale on the T8, $150 + 45 shipping if you are using a Aquirium id think about this as youll never get it spot on.. If thats all you can do for now cover 3 sides with cardboard and it will accually make it hold heat 100% better.
I don't recommend anything I dont have experiance with and that has worked for me....