OK, you will need a basking spot to be at least 95 degrees F. If you can't achieve that with a 60 watt, then you may want to up to a 100 watt (unless you can get a wattage in between). I have an 18x18x24 Exoterra and am using a 100 watt on a dimmer and can get the right temp. Ambient temps in the 80s are good. You're humidity is a bit low. You will need to up that to at least above 70%. A good water holding substrate and misting a couple times a day will help with the humidity too.
-----
Lisa

1.0.0 P.m. grandis (Linus)
3.3.0 P. standingi (Kona and her 5 babies)
R.I.P. Toby (7/5/06) I miss you...