Bearded Dragon heating

Question
I have two bearded dragons in separate cages I read that there basking temperature needs to 100 degrees and air temperature needs to be at 85 degrees I have a hard time getting the tank that hot I have juveniles in 20 gallon long with a flurescent tube and a 75 watt heating light and it still doesn't get that hot from the thermometers are telling me the basking area reads 80 to 85 should I use a undertank heater as a secondary source  haven't gotten one yet but I heard that they are bad and they burn an animal if too close to the bottom of the tank I also use papertowels as substrate so they will be close to the undertank heater.  My lamp can only hold bulbs to 75 watts can I put a 100 watt in it because all I could find around me are 75 watt lamps or 150 lamp holders and I don't think 150 will be a good idea I don't wnat them cooked can you help me?

Answer
BUy the 150 watt fixture and put 100 watt bulbs in it.  Do not put 100 watt bulbs in a 75 watt fixture.  Less is ok, more is not.  You do need to reach that 100 degree mark one way or the other.  UNdertank heating is good, and they do not get hot enough (or shouldn't anyway) to ignite your paper towels.