Don't go by wattage alone. Check the temp with a thermometer to see if it's 88-90F. Use whatever wattage will get it in that range. Adjust the distance of the light from the basking area if necessary, but check the temp with a thermometer (put it on an object the height of your turtle on the spot where he would bask and leave it there for about 20 min.).
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-