BLUE TONGUE SKINKS

Hi!
I have a new baby Blue tongue skink. I am currently housing them in a 20gallon terrarium.(temporary set up, baby is only 2 months old. ) What wattage of bulb should I be using to achieve proper temps. my 50 watt is giving me 83'. I know I need to be a bit warmer. would a 100 watt be too warm?

I use a 100 watt bulb but that heats up the tank to around 110 (which is too hot) so I use a lamp dome with a dimmer.