Also keep in mind that different manufacture's bulbs produce a significantly different ammount of heat. This is something i've been playing with quite a bit trying to adjust temps in my cage since summer temps in my house are drastically different than winter temps. I've tried a few different bulbs from Home Depot and it seems like those ones seem to be cooler compared to the same wattage ZooMed Basking bulbs. Also i've found that the ExoTerra ones are much cooler at the same wattage that the ZooMed ones are.
I have a 48x16x16 tank and i'm currently using a 75 watt ZooMed on side with large rock and a 100 watt ZooMed on the side with a short/flat rock that is about an inch higher than the substrate. This gives me two basking spots surface temps at 115 degrees on either side which is on the high side but my two dragons seem to prefer it when it's warmer. That set up gives me 80-85 degrees ambient temp through out most of the tank once the tank has warmed up. It is in a room that is 75 degrees currently now that it has cooled off here in Northern California, but in the summer months I was using a 50 and 75 watt respectively since I don't have AC and the room temp can jump up to 85-90 degrees. On the hotter days I would turn one of the lights off completely.
Since you have a tank that is 3 inches shorter, I would start off with two 50 watt bulbs and raise or lower the basking spot accordingly. This is just a suggested starting point though since so many factors can affect the temp of the enclosure including room temp, time of day, substrate, furniture, your beardie, full screen top or partial screen and glass, etc. It's not the easiest thing to get right but just make sure you get a good digi thermometer and you'll get it right, even if it takes some time and money invested in bulbs to get it right.