The dimensions of your tank sounds like a 40 gallon breeder. I have to say that a 160 watt MVB is a lot of wattage for that sized tank. With the 100 watt MVB, you would want it a minimum of 12" away but 14 to 16 inches would be safer. BUT with a 160 watt, the distance increases to a minimum of 18" with 20 to 22 inches being safer. I would recommend that you make the distance adjustment asap.
What type of thermometer are you using a round gauge (dial), digital strip, temperature gun, or a digital with a long wire & probe end? If using either the round gauge or digital strip, those can be off as much as 20 degrees. So it's important that you invest in either a temp gun or digital with probe end right away so that you know what the true temps are.
I also wanted to mention that you can't use a dimmer on a mercury vapor bulb.