Well it is difficult to say exactly at the minute as this is the first day the light has been on with the completed setup. I intend to do a few days trial while I am at home over the weekend to see what the temperatures are like.
However at the minute it is looking like I could do with a 26W. The temp readings are:
Cool end - 69.7 F
Warm end - 74.2 F
On the branch under the light - 79 F
Humidity - 68%