Like Brandon said, if you plan on using a MVB, then 24 inches would be good although the 100 watt Mega Ray can go down as low as 12 inches from the basking spot. If you plan on using a fluorescent tube UVB then 18 inches is better unless you plan on building something for him/her to sit up at about 6-8 inches from the tube.
By the way i'm curious as to what Animal Plastics cage you are talking about cause the T12 is 48x24x24 and the T13 is 48x30x18. Are you having them do a custom for you? I personally was planning on the beardie/uro cage which is 48x30x18 since I figured Harley would get more use out of 6 more inches of depth to her cage vs 6 inches of height which is only useable by how many branches or platforms you have above 18 inches.